WO2008059249A1

WO2008059249A1 - Mats

Info

Publication number: WO2008059249A1
Application number: PCT/GB2007/004355
Authority: WO
Inventors: Adam Kelsall
Original assignee: Saffil Limited
Priority date: 2006-11-14
Filing date: 2007-11-14
Publication date: 2008-05-22
Also published as: GB0622652D0

Abstract

A fibre mat for supporting a monolith in an exhaust system of an engine, the mat comprising alumina fibres retained in a binder matrix, the fibres having been heat treated and comprising above 5 w/w % silica and greater than 5 w/w% alpha alumina. The increase in silica leads to an increase in friction coefficient.

Description

Mats

This invention relates to mats and, in particular to mats which can be used to support ceramic, metal or other monoliths in exhaust systems. For example, in apparatus for the catalytic conversion of gases emitted from combustion chambers.

Catalytic converters are used on motor vehicles to reduce the amount of noxious chemicals which are emitted to the atmosphere by catalytically oxidising gases produced during the combustion of fossil fuels. To convert pollutant gases into less atmospherically harmful ones, exhaust gases are passed through a monolith which provides a large heterogeneous catalytic surface to oxidise the pollutant gases. Diesel particulate filters (DPFs) are used on diesel engine powered vehicles to reduce the amount of particulate matter emitted. DPFs may also be arranged to catalyse reduced gases (e.g. CO, hydrocarbons) which would otherwise be emitted.

Exhaust gases are emitted from the combustion chamber and, consequently, are hot. The monolith is located within a metal can, mounted as part of a vehicle's exhaust system, through which the exhaust gases pass. As the engine starts and begins to emit gases, the exhaust system is heated. Monoliths typically have different expansion coefficients to the cans in which they are located, they are also relatively expensive and so the reduction or limitation of damage thereto is an important consideration. To ensure that the monoliths are securely held within the can, mats are provided, e.g. wrapped, around the monoliths prior to their installation in the can. It is necessary for the mats to be fabricated from fibres which can withstand the thermal cycling conditions. The density of the mat must also be controlled to ensure that, as the monolith and can differentially or similarly expand, the pressure exerted on the monolith is not sufficient to cause damage. The mat also protects the monolith from damage cause by vibration of the vehicle. The mat must have an internal laminar shear strength sufficient to hold the monolith during use. The coefficient of friction must be sufficient to retain the monolith in place during use.

Typical prior art mats are formed from inorganic fibres, formed from say alumina or aluminosilicates, held in, say, an organic binder matrix. The organic binder matrix usually comprises about 10% of the total weight of the mat and, as the hot gases pass through the monolith, it is burnt off to leave a purely inorganic mat. The binder ensures that the mat can be handled and the wrapped or otherwise at least partially encompassed monolith can be installed in the can.

The applicant of the current application offer Saffil (RTM) fibres which can and are formed into mats for holding monoliths. Saffil fibres are alumina fibres having between 3 and 5 w/w% SiO₂ present as a phase stabiliser. A typical Saffil (RTM) fibre mat of 3.5 μm mean fibre diameter will have a coefficient of kinetic friction of 0.45 when measured against a polished steel substrate tilt table.

It is an objective of this invention to provide a mat which has improved friction characteristics compared to conventional Saffil (RTM) mats but which does not exhibit concomitant inferior properties or exhibit other deleterious characteristics.

Accordingly, a first aspect of the invention provides a fibre mat for supporting a monolith in an exhaust system of an engine, the mat comprising alumina fibres retained in a binder matrix, the fibres having been heat treated and comprising above 5 w/w % silica and greater than 0.5 w/w% alpha alumina content. A second aspect of the invention provides a method of forming fibres with enhanced friction characteristics, the method comprising forming a solution of an aluminium containing compound and adding a water soluble silicon-containing compound, fibrising the solution to provide fibres, drying and heating treating the fibres, wherein the silica concentration is above 5 w/w % in the fibres and the alpha alumina content is above 0.5 w/w%.

The fibres may be formed by spinning, drawing, blowing, extrusion through a spinneret or combinations thereof.

The fibres are preferably dried, heated at 100-600⁰C for 1 to 60 minutes.

The fibres are preferably heat treated at 1000-1400⁰C, preferably 1100-1400⁰C, and most preferably 1300 to 1400⁰C, for say 10 to 60 minutes.

Typically, the fibres will deliver 1 to 3, say 1.1 to 2.8, preferably 1.2 to 2.5 kgfcm² resiliency at 0.3 gap bulk density (GBD).

The alpha alumina content may be above 0.8 w/w%, e.g. from 0.8 to 15 w/w% or higher. The silica content may be up to, say 20 w/w%, say from 5 to 15 w/w%.

Preferably the silicon-containing compound is a silica sol or one or more siloxanes, preferably comprising at least one water-soluble siloxane, a blend of siloxanes, or a blend of a sol and one or more siloxanes. If used, the silica in the sol is preferably finely dispersed and of a narrow size distribution, preferably at the lower end of the size range, say from 10 to 1000 nm, e.g. 10 to 800 nm.

The silica may be preferentially distributed toward the centre of the fibres. For example, the fibres may have a sheath of alumina, preferably pore free alumina. The alumina may comprise theta alumina. Preferably the sheath may extend to a depth of from 0 to 1000, preferable 0 to 200nm, say 0 to 150 nm, e.g. about 100 nm from the surface of the fibres. In some embodiments the silica may be uniformly distributed across and along the fibre, in other cases there may be a non-uniform distribution.

There is further provided in a third aspect of the invention a method of forming a mat, the method comprising dispersing alumina fibres in a binder solution, the fibres having been heat treated and having above 5 w/w % silica, which is preferentially distributed at or towards the centre of at least some of the fibres, and above 5 w/w% alpha alumina forming the dispersion into a sheet and drying the sheet to provide a mat having improved friction performance.

The aluminium containing compound may be a simple inorganic compound including the hydroxides; the halides and oxyhalides, especially chlorides and oxychlorides; carbonates; nitrates; phosphates; or sulphates. Other salts of organic acids such as neutral or basic acetates, oxalates, propionates, or formates or organo-metallic compounds are also suitable. Basic salts are preferred as they polymerise in solution.

Especially preferred are aluminium oxychloride, basic aluminium acetate, basic aluminium formate, aluminium chlorohydrate. The silicon compound is preferably a compound containing a monomeric or polymeric siloxane, silanol or silanolate group, and/or a water-solubilising carbon functional group. By water-solubilising carbon functional group is meant a group which is attached to the compound through carbon and which confers water-solubility on an otherwise relativey insoluble compound. Examples of such groups are amine, amide, ester alcohol, ether and carboxyl groups. More preferably the silicon compound is selected from water-soluble polysiloxane-polyoxyalkylene copolymers. Such copolymers may conveniently be divided into those in which the polymer blocks have Si-C linkages and those in which the polymer blocks have Si-O-C linkages. Si-C linkages are preferred as copolymers having such linkages are more stable to hydrolysis than those having Si-O- -C linkages.

Examples of water-soluble Si-C linked copolymers useful for the compositions of the invention are described in GB 955,916 and GB 1 ,133,273. Examples of suitable water- soluble Si-O-C linked copolymers are described in GB 954,041.

It is also preferred that the silicon compound be compatible with the other compounds of the liquid composition, especially in not precipitating a gel or a solid therefrom. Thus, silicon compounds which are strongly alkaline in water solution are less satisfactory than those which produce a neutral or acidic reaction in water.

Other silicon compounds which may be used in the compositions include water-soluble alkoxy silanes, quaternary and other water-soluble nitrogen-containing silanes and siloxanes. Alkali metal siliconates are generally sufficiently water-soluble to be useful, for example CH₃Si(OH)(ONa)₂ or CH₃Si(OH)₂(ONa). A selection of polysiloxane copolymers which may be used in this invention are shown in Table 1.

SSlicime to

Λ pp row. pαtyetfeer raliα

Ref. Si[ucsmc MW WW

!;2.3

(CHJ_jTntCO— (PC_jHJ_nOMe

C Wϋ_jSKOSiM*ii)_j.,{OHMe>_tsO-ϊMi], xsm 1*3

D* Mu_:r5røSINft.i),ι.(f}SiMe),0SiMβ, wm 1:2.3

"^" ^ ,N, Ms₁

•dDH^βi Him IKiT rfc₄lM.oc,ttj,i(0t»KAj • Wi

Table 1 - Siloxanes for use in the invention

Additional components such as pigments, polymers, colourants, surfactants, viscosity control additives or source of other oxides, may be included in the compositions.

The compositions may conveniently be prepared by dissolving the metal compound, the silicon compound and any other soluble components in water in any convenient order. For some embodiments it is necessary to provide some heat to assist dissolution. The compounds may be formed from suitable precursors, usually in the presence of the water solvent.

Especially for use of the compositions for making fibres as hereinafter described, a water-soluble silicon-free organic polymer is a much preferred additional component on the compositions. It is postulated that by having a fibre with an increased level of silica it is possible to heat treat the fibre so as to control the crystallinity and thereby increase fibre stiffness and resiliency performance. A control of crystallinity and silica concentration allows for improvements of friction characteristics whilst maintaining fibre resiliency.

5

In this specification the terms 'enhanced friction characteristics', 'improved friction performance' and similar are intended to refer to a friction characteristic that is improved over a Saffil fibre having the same mean fibre diameter (to within one or more significant figures) and substantially the same distribution (i.e. (μ-2σ) ≤ 90% ≤(μ + 2σ)).

I O

The binder may be inorganic or organic or both. Sols or latex, e.g. flocculated latex, systems or both may be used. Alternatively or additionally, thermoplastic polymers may be used and/or fibrillated fibres or other organic pulp.

15 The term w/w% is intended to mean weight of component as measured as a proportion of the overall weight. Thus 10 w/w% of X means that 10% of the overall mass is made up of component X.

In order that the invention may be more fully understood, reference is made to the0 following examples.

Example 1

A solution of aluminium chlorohydrate and polysiloxane copolymer A was made and5 the resulting solution spun to produce fibres. Sufficient siloxane was added to give the disclosed target formulation. The fibres were dried and heat treated at 135O⁰C for 20 to 40 minutes. Each sample was analysed and then formed into a 50 cm² pad with a basis weight of 1250 gm^"2 placed on a tilt table with coarse steel tilt plate and the coefficient of friction (COF) measured.

The results are shown in Table 2.

Description % χ-At₂O₃ "FD(μm) % improvement in COF^C

4% SiO₂ ^a 2.3 5.1

7% SiO₂ 3.2 5JJ 9

Table 2 - Results for Example 1 a 4% SiO₂ is standard Saffil (RTM) fibres

FD is the median fibre diameter c COF is the coefficient of friction

Comparative Example 1

A standard Saffil (RTM) solution was spun to provide fibres having 4% SiO₂, 3.5 μm median fibre diameter.

A pad was formed as per Example 1 above and the COF was measured against the same coarse plate. The fibre of this Example appeared to give a slightly improved COF over the mat made from the standard solution in Example 1.

Whilst we do not wish or intend to be bound by any particular theory, we understand that the increase in COF experienced when reducing the fibre diameter is due to the relatively larger number of ends which are able to interact with a plate in the smaller diameter fibre mat than in the larger diameter fibre mat for a given mat area. Example 2

A fibre was spun from a solution of aluminium chlorohydrate and polysiloxane copolymer A to provide a fibre with 8.5% SiO₂ and median fibre diameter of 3.7 μm.

The fibres were dried and heat treated at about 1320⁰C for 20 minutes.

The fibre was formed into a mat and the measured COF gave a 20% increase over a mat formed from standard Saffil fibres of an equivalent fibre diameter and distribution.

Example 3

An equivalent experiment to that of Example 2 was carried out except ^ιwith fibres having a median fibre diameter of 3.4μm.

The resulting coefficient of friction was over 10% higher than equivalent Saffil product.

Example 4 A series of experiments were run with target SiO₂ to provide about 5 to 7% silica in the final fibre.

A solution of aluminium oxychloride and different amounts of siloxane copolymer B were spun to provide fibres with different silica concentrations.

The results are shown in Table 3. Sample % SiO₂" % Increase in COF

CE2^e 3.4 0

1 4.7 2.5

2 5.8 8

3 5.9 10

4 6.9 14

Table 3 - Results for Example 4 d The amount of retained silica was measured by XRF as a w/w% OfAi₂O₃. e The denomination CE means Comparative Example

The results are shown in the graph of Figure 1A which provides a graph of increase in COF against percentage amount of silica in the fibre..

In each case, the fibres were heat treated at 1150 to 1400⁰C for between 10 and 30 minutes after having previously been dried.

The amount of alpha alumina in each case was above 0.5 w/w% as measured by x-ray diffraction.

)

Sample 1 was, within the limit of error, the same as standard Saffil material. Indeed, it would appear that improvements are seen only once the amount of retained silica is above 5%.

Figure 1 B shows the effect of increased silica concentration and heat treatment on the coefficient of friction of wet formed pads without binders against a 409 Stainless steel tilt table. As will be appreciated, as the proportion of silica increases, the coefficient of friction increases.

Example 5

5 The spinning solutions of Samples 2, 3 and 4 of Example 4 were run on a plant scale trial.

Fibre sheets of 1250 gm^"2 material were formed as 50 cm² pads.

io The resultant fibre pads were measured for their coefficients of friction against a 304 stainless steel plate with an 1165g weight placed on the top surface.

In each case the coefficient of friction showed a 10-15% improvement over the equivalent Saffil fibre.

I5

The invention will now be further described, again by way of example only, and with reference to Figures 2 and 3, in which:

Figure 2 is a graph showing the relationship between coefficient of friction and »o silica content in mats according to the invention; and

Figure 3 is a graph showing the relationship between coefficient of friction and fibre diameter for prior art mats and mats of the invention.

15 Figure 2 shows the effect of increasing silica content of fibres made into mats. Line P shows that for fibres having a diameter of 5.5 μm, the coefficient of friction increases with silica content when measured against exhaust can steel. Line Q shows that the coefficient of friction increases with silica content for a mat made with fibres having a mean diameter of 3.5 μm, when measured against a polished steel surface on a tilt table and line R shows that the coefficient of friction increases with silica content for a mat made with fibres having a mean diameter of 8 μm, when measured against a polished steel surface on a tilt table.

Figure 3 shows the relationship between coefficient of friction and fibre diameter for a prior art mat made with Saffil fibres having 4 w/w% SiO₂ (line S) and for a mat having enhanced silica content by increasing the level of siloxane present in the spinning solution to provide fibres with above 5 w/w% silica (line T).

Example 6

In order to measure the coefficient of friction at the high compressive levels during use a larger tilt table was constructed using a 304 stainless steel friction plate.

A sample of high silica fibre (having 8 w/w% SiO₂) was formed into a 50 cm² pad with a basis weight of 1250 gm^'2 and the coefficient of friction was measured for increasing loads (up to 969 gem^'2).

The results are shown in Figure 4, which is a graph of relative increase in COF (as compared to equivalent Saffil pads) to load applied.

As shown in Figure 5, which is a section of the graph of Figure 4, the graph from 400 to 969 g.cm^"2 is substantially linear. Example 7

In order to determine the improvement at high applied loads the following fibres were formed, made into mats having the same physical characteristics and tested against a 304 stainless steel plate substrate with an applied load of 969 gem^"2.

Fibre Mean Diameter/μm COF % increase

CE4 3.1 0.29 0

CE5 3.7 0.32 0

Sample 6 3.1 0.31 7

Sample 7 3.7 0.4 25

Table 4 - Results for Example 7

The Samples 6 and 7 fibres had a silicon concentration of 7 to 9 w/w % as compared to a silica concentration of 3 to 5 w/w % for the Sample CE4 and CE5 fibres.

The data demonstrate that the fibres made with increased silica have significantly higher coefficients of friction at highest levels, when compared to a prior art fibre.

It will also be appreciated that at usual initial 'canning' loads (i.e. about 960 g.cm^'2), the high silica mats show improvement over standard Saffil mats when the fibre diameter is optimised for a substrate. Example 8

It is known that different substrates have different surface roughnesses. Therefore a given fibre mat will exhibit different coefficients of friction depending on which substrate it is measured against.

In order to determine optimum fibres for any particular substrate, a series of substrates were used to optimise fibre diameter for a given silica level of 7 to 9 w/w %. The results are shown in Table 5.

Substrate ^e Fibre Diameter / μm ^f Improvement / %

304 SS 3 to 4 15 - 25

Polished 304 SS 3 to 4 10 - 20

409 SS 3.5 to 5.5 10 - 20

429 SS 3.0 to 3.5 10 - 20

Table 5 - Results for Example 8 eThe fibre diameter is given as a mean value.

Improvement is the percentage improvement over a corresponding Saffil prior art fibre mat.

As can be seen a significant improvement is demonstrated for various substrates as compared to the prior art Saffil fibres.

It is known that pressure performance for mats in catalytic converters decreases over time.

By combining the data in Figures 4 and 5, together with decay information over thermal cycling regimes, it is possible to show the performance improvement of a mat of the invention compared to a prior art mat. Figure 6 is a graph of increase of COF against pressure cycle.

Referring to Figure 6 it can be seen that after the 100^th cycle a mat of the invention consistently outperforms a standard Saffil mat. It is postulated that the increase in friction at lower compressive loads compensates for any or a reduction in pressure performance, a factor not experienced with standard Saffil mats.

Whilst we do not wish to be bound by any particular theory, we further believe that the increase in the coefficient of friction is due at least in part to increased modulus of the fibres caused by the increased stiffness of the fibres.

As has been stated, by increasing silica in the fibres it is possible to heat treat the fibres to control the crystallinity.

Figure 7 provides a graph of resiliency against temperature of heat treatment.

In Figure 7 mat formed using fibres of the invention (line U) having 14% silica and one made using prior art fibres (having 4 w/w% silica) are compared. As can be seen, the mat of the invention has much higher resiliency levels than the prior art mat. Whilst performance decays as higher heat treatment regimes are utilised, the mats of the invention have improved performance even at the high temperatures used.

Figure 8 provides a graph of variation in Coefficient of Friction with changing alpha alumina concentrations.

In Figure 8 the effect of altering the alpha alumina concentration on the coefficient of friction are shown. As the amount of alpha alumina increases above 5w/w% the coefficient of friction increases greatly. This is ascribed to the higher stiffness of the fibres.

Example 9

In order to establish whether or not there was a silica concentration gradient through the fibres, e.g. a higher concentration at the periphery or centre of fibres dynamic SIMS depth profile studies were conducted which showed that in some cases silica concentration became apparent at a depth of about 100nm from the surface of the fibre.

Figure 9A is a transmission electron microscope image of a prior art fibre.

Figure 9B is a transmission electron microscope image of a fibre according to the invention.

Referring to Figure 9A, there is shown a fibre having a fine even grain structure formed predominantly of delta alumina with some theta and very low levels of alpha alumina (e.g. 1 to 2 %).

By contrast, Figure 9B shows a fine even core but a denser outer ring containing increased levels of alpha alumina.

Other fibres of the invention did not appear to exhibit this structure.

Figure 1OA is an XRD spectrum of prior art fibres. Figure 10B is an XRD spectrum of fibres of the invention.

As can be seen from Figures 1OA and 10B, the fibres of the invention display greater amounts of alpha and theta alumina but reduced levels of delta alumina, compared to the prior art.

Whilst we do not wish to be limited by any particular theory we believe that the use of relatively high levels of siloxanes and/or alternative silica sources allows the fibre to be heat treated in such a way as to control the crystallinity and thereby allow development of alpha alumina. The fibres demonstrate an increase of stiffness and/or improved resiliency leading to an improved performance.

Claims

1. A fibre mat for supporting a monolith in an exhaust system of an engine, the mat comprising alumina fibres retained in a binder matrix, the fibres having been heat treated and comprising above 5 w/w % silica and greater than

0.5 w/w% alpha alumina.

2. A mat according to Claim 1 , wherein the fibres comprise above 5 to 18 w/w % silica.

3. A mat according to Claim 1 or 2, wherein the fibres have above 5 w/w% alpha alumina.

4. A mat according to Claim 1 , 2 or 3, wherein the fibres have been heat treated at a temperature of 1100 to 1400 ⁰C.

5. A mat according to Claim 4, wherein the fibres have been heat treated for from 10 to 60 minutes.

6. A mat according to any of Claims 1 to 5, wherein, subsequent to burn out of the binder, the mat exhibits a resiliency of 1 to 3, say 1.1 to 2.8, preferably 1.2 to 2.5 kgfcm² resiliency at 0.3 gap bulk density (GBD).

7. A mat according to any preceding Claim, wherein the fibre comprises theta alumina.

8. A mat according to any preceding Claim, wherein the silica is preferentially distributed towards the centre of the fibre.

9. A mat according to Claim 8, wherein the periphery of the fibre is relatively silica depleted to a depth of from 0 to 200nm, optionally 0 to 150 nm, e.g. about 100 nm from the surface of the fibres.

10. A method of forming fibres with enhanced friction characteristics, the method comprising forming a solution of an aluminium containing compound and adding a water soluble silicon-containing compound, fibrising the solution to provide fibres, drying and heat treating the fibres, wherein the silica concentration is above 5 w/w % in the fibres and the alpha alumina is above 5 w/w%.

11. A method according to Claim 10, wherein the fibres are formed by one of spinning, drawing, blowing, extrusion through a spinneret or combinations thereof.

12. A method according to Claim 10 or 11 , further comprising drying and heating the fibres at 100-600⁰C for 1 to 60 minutes and then heat treating.

13. A method according to Claim 10, 11 or 12, comprising heat treating the fibres at 1000-1400⁰C, preferably 1100-1400⁰C.

14. A method according to any one of Claims 10 to 13, comprising heat treating the fibres for 10 to 60 minutes.

15. A method according to any one of Claims 10 to 14, wherein the silicon- containing compound is a silica sol or one or more siloxanes, or a blend thereof.

16. A method according to Claim 15, wherein the, one or more siioxanes are water- soluble.

17. A method according to any of Claims 10 to 16, wherein the silicon-containing compound is a silica sol which is finely dispersed and of a narrow size distribution.

18. A method according to Claim 17, wherein the silica sol has a size distribution at the lower end of the size range.

19. A method of forming a mat, the method comprising dispersing alumina fibres in a binder solution, the fibres having been heat treated and having above 5 w/w% silica and above 0.5 w/w% alpha alumina, forming the dispersion into a sheet and drying the sheet to provide a mat having improved friction performance.

20. A fibre mat for supporting a monolith in an exhaust system of an engine, the mat comprising fibres retained in a binder matrix, wherein at least some of the fibres have: a) above 5 w/w % silica, preferably distributed at or towards the centre of the fibres; b) above 0.5 w/w% alpha alumina c) both.