GB2428604A

GB2428604A - Fluorosiloxane anti-foul coating on heat exchanger

Info

Publication number: GB2428604A
Application number: GB0516142A
Authority: GB
Inventors: Stephen John Joyce; Christopher David Whelan; Alec Gunner; Alan Taylor
Original assignee: Visteon Global Technologies Inc
Current assignee: Visteon Global Technologies Inc
Priority date: 2005-08-05
Filing date: 2005-08-05
Publication date: 2007-02-07
Anticipated expiration: 2025-08-05
Also published as: GB0516142D0; GB2428604B

Abstract

A heat exchanger 1 has a coating 34 to reduce fouling caused by the adherence of particulates to the surfaces 30, 32 of the heat exchanger 1. The coating comprises a siloxane based network, the network comprising silicon atoms, oxygen atoms and fluorinated alkyl groups. Each silicon atom in the network is coupled to another silicon atom through siloxane linkages, and a plurality of the silicon atoms are also bonded to a fluorinated alkyl group. The fluorinated alkyl groups are distributed substantially evenly throughout the bulk of the network. The coating is particularly suitable for use in a combined exhaust gas recirculation and charge air cooler for a vehicle combustion engine.

Description

ANTI-FOULING COATING

Field of the Invention

The present invention relates to an apparatus and method for controlling fouling in a heat exchanger which is exposed to combustion products, particularly although not exclusively for use in a diesel engine of a vehicle.

Background of the Invention

Fouling is a common problem associated with heat exchangers. The term fouling' generally relates to any kind of deposit that forms on the heat transfer surfaces of a heat exchanger.

For heat exchangers which are exposed to combustion products, a significant type of fouling is particulate fouling'. Particulate fouling is primarily caused by the adherence of hydrocarbons to the internal surfaces of the heat exchanger, thus causing the build up of a soot layer.

Corrosion fouling' and chemical reaction fouling' also tend to affect heat exchangers which are exposed to combustion products. These heat exchangers tend to operate at high temperatures, and the chemical species comprising the combustion products may react with the surfaces of the heat exchanger, or cause the heat exchanger to corrode.

These types of fouling generally cause a less thermally conductive layer to form on the heat transfer surfaces.

This layer tends to adversely affect the heat transfer properties of the surfaces. In addition to affecting the heat transfer properties of the heat transfer surfaces, corrosion fouling also degrades the materials comprising the heat exchanger.

In addition to affecting the thermal properties of heat transfer surfaces, fouling of a heat exchanger, particularly of the heat exchange tubes and cooling fins, tends to restrict the airflow therethrough, resulting in pressure drops. In general, fouling leads to a reduction in the overall efficiency of the heat exchanger. Fouled surfaces therefore need to be regularly cleaned or replaced to maintain heat exchanger efficiency. This is often impractical and costly.

To reduce some of the fouling problems associated with heat exchangers, it is known to provide a coating on a heat exchanger. United States patent application US 2001/0054500 Al discloses a sol-gel coating for a titanium heat exchanger. The purpose of this coating is to protect the heat exchanger from corrosion fouling.

Sol gel techniques allow a thin layer of material to be deposited on a surface at moderate temperatures. Silicon- based materials have been widely used in sol-gel coatings.

These coatings are typically made by colloidal sol-gel techniques, in which silica particles coalesce and ultimately gel to form an extensive silica network.

The problem with colloidal sol-gel techniques is that the coatings formed by this method tend to be weak and prone to cracking. This is due, in particular, to low levels of cross-linking. Low levels of cross linking also results in coatings with poor surface coverage of substrate.

A polymeric sol-gel technique is an alternative to the colloidal technique. Coatings provided by way of the polymeric technique have higher levels of cross linking, and are therefore stronger and less prone to cracking than conventional particulate-based materials. Furthermore, coatings provided by the polymeric sol-gel technique have a better substrate surface coverage than coatings provided by the colloidal technique.

A polymeric sol gel technique is described in patent document WO 01/25343 Al. The silane based sol-gel coatings disclosed in this document form an inert barrier that protects the underlying substrate from corrosion fouling. The coatings disclosed in WO 01/25343 Al are generally manufactured from three main precursor materials. These precursors consist of two different inorganic monomers which can be hydrolysed to form an inorganic sol, and a polymerisable organic species. The resulting coating has an inorganic phase and an organic phase; these two phases forming interpenetrating networks on the nanometre scale. The role of the organic component is to increase the structural integrity of the network allowing thick coatings to be formed.

The above discussed prior art discloses sol-gel coatings that protect the underlying substrate from corrosion fouling. There remains a need for a coating that can additionally protect the substrate from particulate fouling. Such a coating has application in heat exchangers which are exposed in use to combustion products.

Summary of the Invention

According to a first aspect of the present invention, there is provided a heat exchanger for use in an exhaust gas recirculatjon system of a vehicle, the heat exchanger having a coating on at least part of a surface, the coating comprising of a siloxane based network, the network comprising silicon atoms, oxygen atoms and fluorinated alkyl groups, with each silicon atom being coupled to another silicon atom through siloxane linkages, wherein a plurality of the silicon atoms are also bonded to a fluorinated alkyl group, the fluorinated alkyl groups being distributed substantially evenly throughout the bulk of the network.

The coating significantly reduces fouling in a heat exchanger exposed to combustion products. The coating forms a continuous glass-like layer on the surface of the heat exchanger to protect the surface from corrosion fouling. The coating also prevents the surfaces of the heat exchanger from being exposed to chemicals in the combustion products thus reducing chemical reaction fouling. A preferred embodiment of the coating is particularly inert and is not prone to chemical reaction fouling to an appreciable extent.

The coating has a very low surface energy due to the presence of fluorine. This low surface energy greatly reduces the ability of species in the combustion products to adhere to the coating. The reduced adherence properties of this coating promotes a self regeneration of the heat exchanger as the soot layer builds up and then falls off in a cyclic fashion. The coating therefore solves the problems associated with particulate fouling in a heat exchanger and means that the heat exchanger surfaces do not require periodic maintenance or replacement.

The coating has many advantageous over other well known fluorinated coatings for example PTFE. The high levels of fluorination in PTFE make it impossible to bond chemically to a substrate. PTFE coatings are held to the substrate by mechanical force, which requires extensive preparation of the substrate, for example the substrate may need to be roughened. This is not appropriate for many heat exchangers as such treatments can affect the thermal properties of the heat transfer surfaces. The level of fluorination in the coatings provided by the present invention is relatively low. Furthermore, the fluorine is distributed throughout the bulk of the coating and not just at the surface of the coating. This enables the coating to bond chemically to the substrate by silicon- oxygen bonds and does not therefore require mechanical preparation of the substrate. The advantage of having fluorine distributed throughout the bulk is that the coating retains its anti-fouling properties even if it wears down.

Preferably, the coating also comprises organic moieties bonded to a plurality of the silicon atoms, the organic moieties being distributed substantially evenly throughout the bulk of the network. Up to 30% of the silicon atoms in the network may be bonded to an organic moiety, and between 0.1 and 2% of the silicon atoms may be bonded to a fluorinated alkyl group. Preferably, about 24% of the silicon atoms in the network are bonded to an organic moiety, and about 1% of the silicon atoms are bonded to a fluorinated alkyl group. Particularly effective coatings have been produced where the organic moieties are methacrylate groups and the fluorinated alkyl groups have a carbon chain length of 8 to 10 carbon atoms. However, effective coatings may also be produced where the organic moieties are other relatively bulky non-hydrolysable ligands and the fluorinated alkyl groups may have a carbon atom chain length in the range of 2 to 12 atoms.

Preferably the surface of the heat exchanger is made from aluminium or an aluminium alloy, however the heat exchanger may have plastic end caps or plastic header units, in which case the surface may be made from a plastics material. The surface may be an internal surface of the heat exchanger, or it may be an external surface.

The coating can bond to both metal and plastic. The coating may be applied to component parts of the heat exchanger before the heat exchanger is assembled.

Alternatively, the coating may be applied to the entire heat exchanger after assembly.

It is important that the coating has a minimal effect on the heat transfer properties and the efficiency of the heat exchanger. This may be achieved by ensuring that the coating is a thin layer. If the coating is too thick, it is prone to cracking as the heat exchanger gets hot and expands. If the coating is too thin, problems associated with poor surface coverage may be encountered. It should be understood that different surfaces of the heat exchanger may be coated with coatings of different thicknesses, and the thickness of the coating may not be constant over a given area. The average thickness of the coating is preferably in the range 0.5 to 2 pm across the greater part of its area. Such thicknesses provide a good surface coverage and will have a negligible effect on the heat transfer properties of the heat exchanger.

According to a second aspect of the present invention there is provided a method of coating a surface of a heat exchanger with an anti-fouling coating, the method comprising the steps of: hydrolysing network monomers to form hydrolysed network monomers; hydrolysing anti-fouling monomers to form hydrolysed anti-fouling monomers; combining and mixing at least the hydrolysed network monomers and the hydrolysed anti-fouling monomers to form a solution of hydrolysed monomers; allowing the hydrolysed monomers to polymerize to form a sol; applying the sol to a surface of a heat exchanger; curing the sol to form a hard coating on the surface of the heat exchanger; wherein the network monomers have the general formula: SIR1aR2b (OR3) c [1] wherein R' and R2 are different organic moieties, R3 is an organic moiety, a and b are integers in the range 0 to 2, and c is an integer in the range 2 to 4, such that (a+b+c) equals 4; wherein the anti-fouling monomers have the general formula: S R4dR5e(0R6)f [2] wherein R4 is a fluorinated alkyl group, R5 and R6 are organic moieties, d is an integer in the range 1 to 3, e is an integer in the range 0 to 2, and f is an integer in the range 1 to 3, such that (d+e+f) equals 4.

A common problem associated with sol gel coatings produced using polymeric techniques is that they are prone to cracking at thicknesses above about 100 nanometres (nm) This is due to very high levels of cross- linking in the network making the coatings brittle. Coatings with thicknesses of around 100 nm do not provide a good surface coverage. The coatings provided by the present invention can be made with thicknesses of up to around 2 p.m without cracking. This is because precursor monomers are used that have some non-hydrolysable groups. The presence of non-hydrolysable groups means that there are fewer reactive sites at which polymerisation can occur. Fewer reactive sites means coatings with lower levels of cross linking may be produced. This results in less brittle coatings which are not prone to cracking at thicknesses up to about 2 mt.

Structural monomers may also be included in the coating to modify the coatings properties and in particular make the coating more flexible. The method may also therefore involve hydrolysing structural monomers to form hydrolysed structural monomers. In which case, the method step of combining and mixing at least the hydrolysed network monomers and the hydrolysed anti-fouling monomers may further include combining and mixing the hydrolysed structural monomers to form the solution of hydrolysed monomers.

The structural monomers have the general formula: SIR7gR8h(0R9)k [3] - wherein R7 is a non-hydrolysable organic moiety, R8 and R9 are organic moieties, g is an integer in the range 1 to 3, h is an integer in the range 0 to 2, and k is an integer in the range 1 to 3, such that (g+h+k) equals 4, and wherein the structural monomers are not the same species as the network monomers.

The requirement that g must be in the range 1 to 3, means that the structural monomers must contain a non- hydrolysable organic moiety. This moiety will remain attached to the silicon atom after hydrolysis, and results in the structural monomer having less reactive sites at which polymerisation can occur. This reduces cross- linking in the network, allowing a more flexible coating to be made that is not prone to cracking.

The network monomers will comprise the majority of the precursor monomers in the solution of hydrolysed monomers, however, preferably, about 2 to 30 % of the monomers comprising the solution are hydrolysed structural monomers, and about 0.1 to 2% of the monomers are hydrolysed anti-fouling monomers. Preferably, the molar ratio of network monomers: structural monomers: anti- fouling monomers is substantially 75:24:1.

A preferred species for the network monomers is tetraethoxysilane (TEOS), and preferred species for the structural and anti-fouling monomers are 3(trimethoxysilyl) propylmethacrylate (MPTMA) and 1H, 1H, 2H, 2H-perfluorooctyltriethoxysilane (PFOTES) - 11 - respectively.

The sol may be aged prior to the sol being applied to the surface. The ageing promotes the formation of the inorganic network. Preferably, the sol is aged by heating the sol at a temperature in the range 60 to 90 C, for about 1 to 48 hours.

Preferably, the sol is applied to the surface of the heat exchanger by immersing the surface in a tank of sol and draining the sol from the tank at a predetermined rate.

The rate of draining and the viscosity of the sol determine the thickness of the coating. This is known as the dip method'.

It may be advantageous to coat the entire heat exchanger, including the external surfaces, with the coating. This is because the coating not only has anti-particulate fouling qualities, but it also prevents the surfaces from corroding. Corrosion is a problem that may affect any surface of the heat exchanger. The dip method is a particularly simple and effective method of coating many surfaces simultaneously. The coating method may therefore also include the steps of assembling the heat exchanger from its component parts prior to the coating being applied. The inside of the heat exchanger may then be filled with sol and allowed to drain such that a plurality of internal surfaces of the heat exchanger are coated.

Alternatively, the entire heat exchanger may be immersed in sol such that all the internal and external surfaces of - 12 - the heat exchanger are coated.

Once applied to the heat exchanger surfaces, the coating is cured to form a hard layer. Preferably, the coating is cured at a temperature in the range of 150 to 250 C. The curing process may last up to 18 hours, however the coating is preferably cured for between 5 and 30 minutes.

The coating is suitable for coating a heat exchanger for use in a combustion product circuit of a vehicle having an internal combustion engine. The coating is particularly suitable for coating a combined exhaust gas recirculation (EGR) and charge air cooler for a vehicle.

Many modern automotive engines are turbocharged & intercooled, in order to achieve high specific powers.

These engines also tend to employ EGR to reduce gaseous emissions. Two types of intake-system-related heat exchangers are therefore needed in such engines: an intercooler for cooling the charge air, and a separate cooler for cooling the EGR. It is advantageous to use a single heat exchanger that performs the combined EGR & charge air cooling functions. Such a system is hereafter referred to as a combined cooler.

A combined cooler offers substantial cost, weight & package reductions when compared to current systems which utilise separate heat exchangers for cooling the charge air and the EGR. Furthermore, a combined cooler provides lower intake temperature, which gives further potential for improved emission control.

- 13 - The implementation of a combined cooler has proved problematic due to particulate fouling caused by the adherence of hydrocarbons in the recirculated exhaust gases to the internal surfaces of the cooler. As the exhaust gases can contain corrosive substances such as sulphuric acid, the combined cooler is also prone to corrosion fouling.

Under heavy load conditions the exhaust gas can reach temperatures of up to 650 C and the charge air can reach temperatures of up to 250 C. However, exhaust gas recirculation is switched off under heavy load conditions.

At part-load conditions the exhaust gas may reach 400 C, but as the proportion to be recirculated is quite low, and as the charge air temperature is lower than under heavy- load conditions when mixed the temperature of the combined gases is less than 250 C. Therefore, in general, the combination of the gases results in a combined gas with a temperature of less than 250 C. The coating is capable of durable operation at temperatures up to 250 C since the physical and chemical properties of the coating are, substantially unaffected by such temperatures. This makes the coating particularly suitable for use in a combined EGR and charge air cooler.

A combined cooler type heat exchanger for use in a motor vehicle is preferably made from aluminium and may also have plastic components such as plastic headers and end caps. The coating can bond to metal and plastic, enabling the entire cooler to be coated after assembly.

- 14 - Alternatively, component parts of the heat exchanger may be coated individually prior to assembly. The combined cooler is preferably fabricated from aluminium or an aluminium alloy, but other materials e.g. plastic, could equally well be used.

The anti-fouling coating provided by the present invention solves the fouling problems associated with a combined cooler.

The coating may also be applied to internal surfaces of an aluminium exhaust system of a vehicle to reduce the fouling thereof.

- 15 -

Brief Description of the Drawings

The invention will now be further described, by way of example, with reference to the following drawings in which: Figure 1 is a block diagram of a boosted internal combustion engine having a combined EGR & charge air cooler.

Figure 2 illustrates a combined EGR and charge air cooler having a coating on the internal surfaces to control fouling.

Figure 3 is a flow diagram showing the manufacture of sol-gel coatings using a polymeric sol-gel process.

Figure 4 is a graph illustrating the cumulative weight loss of soot covered aluminium test plaques when exposed to an air flow.

Figure 5 is a bar graph illustrating the percentage weight loss of soot covered aluminium test plaques after being exposed to an air flow.

Detailed Description

Figure 1 illustrates a turbocharged system for a diesel engine utilising EGR and having a combined cooler 1 for cooling the EGR and the charge air. The system has a - 16 - combustion chamber 2; exhaust gases 4 from the combustion chamber 2 drive a turbine wheel 6 which drives rotary compressor 8. The exhaust gases 4 from the combustion chamber 2 are combined with compressed air 10, referred to herein as charge air, prior to admission to the combined cooler 1 where the combined gas is cooled.

A typical combined cooler 1 is illustrated in more detail in Figure 2. A combined mixture of exhaust gases and charge air 12 is introduced through an air inlet 14. The combined cooler comprises a cylindrical shell 16 containing a plurality of heat exchange tubes 18 through which the combined mixture of exhaust gases and charge air 12 flows before exiting through an air outlet 20.

The combined cooler shown is water cooled. A coolant inlet 22 allows engine coolant to flow inside the shell 16 and around the tubes 18 prior to egress via an outlet 24.

The coolant enters the heat exchanger in the direction of arrow 26 and exits in the direction of arrow 28. The combined cooler could also be cooled by air, although this embodiment is not illustrated.

The combined cooler has a number of internal surfaces which are exposed in use to exhaust gases. The internal surfaces include the internal surface 30 of the shell 16, and the internal surfaces 32 of the heat exchanger tubes 18. A coating 34 is disposed on the internal surfaces and is shown generally by the dotted region.

- 17 - Preferably, the combined cooler is manufactured from aluminium or an aluminium alloy. The combined cooler may also include a header unit and end caps (not shown) made from a plastics material. The internal surfaces of the heat exchanger may therefore include surfaces of the plastic end caps or header unit, and the coating may also be applied to these surfaces.

The coating comprises a siloxane based network. In this example, the network is formed by the hydrolysis and polycondensation of at least two different types of precursor monomers.

The first type of precursor monomers are referred to herein as network monomers'. The role of the network monomers is to polymerise to form the coating's primary network. The network monomers have the general formula: SR1aR2b(OR3)c [1] wherein R' and R2 are typically independently selected from organic moieties having 1 to 10 carbon atoms, and which may contain an ether linkage or ester linkage; R3 is an organic moiety having 1 to 10 carbon atoms; wherein a and b are independently selected from zero and integers in the range 0 to 2, c is independently selected from integers in the range 2 to 4, such that (a+b+c) equals 4, where 4 is the valency of Si. For the purposes of this specification, compounds as described above, and having - 18 - the general formula [1] are known as alkoxysilanes.

The network monomers preferably contain at least three hydrolysable groups. Most preferably, the network monomers have four hydrolysable groups and are defined by the general formula [1] in which a=b=O, such that the network monomers are represented by the general formula: Si (OR') 4 [4] In other words, the network monomers preferably contain only hydrolysable ligands bonded to Si. Examples of these compounds include silicon-based inorganic alkoxides such as: silicon tetra-alkoxides such as tetramethoxysilane, tetraisopropoxysilane, and tetrabutoxysilane. A particularly preferred material is tetraethoxysilane (TEOS) (also known as tetraethyl orthosilicate), which has the chemical formula: Si(OCH2CH3)4 [5] If desired, a number of different types of network monomer of general formula [1] may be included in the coating composition.

The second type of precursor monomers are referred to herein as anti-fouling monomers'. The role of the anti- fouling monomers is to modify the primary network so that the coating has anti-particulate fouling qualities. The anti-fouling monomers should contain at least one non- - 19 - hydrolysable group. When the monomers are hydrolysed, this group will remain bonded to the silicon atom. The network will therefore contain some silicon atoms bonded to non-hydrolysable groups derived from the anti-fouling monomers.

To provide the anti-particulate fouling qualities, at least one of the non-hydrolysable groups is a fluorinated alkyl chain. The presence of fluorine in the coating results in the coating having a high contact angle and a low surface energy. Such a surface is energetically unfavourable towards adsorption of particulates and other species, resulting in anti-fouling properties.

The anti-fouling monomers have the general formula: SR4dR5e (OR6) [2] wherein R4 is a fluorinated alkyl group, R5 and R6 are organic moieties, d is an integer in the range 1 to 3, e is an integer in the range 0 to 2, and f is an integer in the range 1 to 3, such that (d+e+f) equals 4.

Examples of suitable anti-fouling monomers include (3,3,3trifluoropropyl) trirnethoxysilane, and perfluorodecyltriethoxysilane (PFDTES) A particularly preferred anti-fouling monomer is 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES), which has the chemical formula: - 20 - CF3(CF2)5(CH2)2Si(OC2H5)3 [6) Although the anti-fouling monomers must include a fluorinated alkyl group, the other functional groups may be selected to provide the coating with additional properties. Many sol-gel based coatings are prone to cracking. Careful selection of the other functional groups, can provide a coating which is more flexible and less prone to cracking. If desired, a number of different types of anti- fouling monomer of general formula [2] may be included in the coating composition.

A third type of precursor monomer may also be included in the coating composition to give the network structural integrity. These monomers are referred to herein as structural monomers'. The structural monomers must contain at least one non-hydrolysable organic group, and it is this group that improves the structural integrity of the network, resulting in a more flexible coating which is less prone to cracking as the heat exchanger expands with increasing temperature. By using separate anti- fouling monomers and structural monomers, the coating's anti- fouling and structural properties may be independently controlled.

The inclusion of a non-hydrolysable group in the structural monomer means that the silicon atom can only form a maximum of three bonds to oxygen atoms in the network, since the silicon remains bonded to the non- - 21 hydrolysable group after hydrolysis. This results in a less brittle network having a greater degree of flexibility. Such a coating is less likely to crack.

The structural monomers have the general formula: SiR7gR8h(0R9) k [3] wherein R7 and R8 are organic moieties, R9 is a non- hydrolysable organic moiety; g is an integer in the range 1 to 3, h is an integer in the range 0 to 2, and k is an integer in the range 1 to 3, such that (g+h+k) equals 4, and wherein the structural monomers are not the same species as the network monomers.

Suitable compounds for use as the structural monomers are those having at least one relatively bulky non- hydrolysable ligand. By relatively bulky typically we mean that the ligand provides greater steric hindrance than a single vinyl group. Examples of such compounds are: 1) (alkyl) alkoxysilanes having an epoxy group such as 3glycidoxypropyltrimethoxysilafle, 3-glycidoxypropyl- triethoxysilane, 3-glycidoxypropylmethyldi-methOXYSilafle, 3-glycidoxypropylmethydiethoxysilane, 3-glycidoxypropyldimethylethoxysilane, 2- (3, 4-'epoxycyclohexyl) ethyltrimethoxysilane, and 3, 4-epoxybutyltrimethoxysilane; ii) (alkyl) alkoxysilanes having an amino group such as N2- (aminoethyl) -3aminopropyltriethoxysilafle, N-2 (aminoethyl) -3-aminopropylmethyldimethOXYsilane, and - 22 Nphenyl-3-aminopropyltrimethoxysilane; iii) (alkyl) alkoxysilanes having an acrylate or methacrylate group such as 3-methacryloxypropyltriethoxysilane, 3-methacryloxypropylmethyldimethylsilane and 3acryloxypropyltriethoxysilane.

Preferred structural monomers include 3- glycidoxypropyltrimethoxysiliane (GPTS), and N-phenyl-3- aminopropyltrimethoxysilane (PAPMS) . A particularly preferred compound is 3- (trimethoxysilyl)propylmethacrylate, also known as 3- (methacryloyloxy)propyl]trimethoxysilane, (MPTMA) . MPTMA has the chemcical formula: 0 OCH3

II I

H20=C-C-OCH2CH2CH2 -Si -00H3 CH3 OCH3 [7] If desired, a number of different types of structural monomer of general formula [3] may be included in the coating composition.

The fluorinated alkoxysilanes comprising the anti-fouling monomers tend to be expensive. MPTMA and other possible structural monomers on the other hand are relatively cheap. By using separate anti-fouling and structural monomers to provide the coating with anti particulate fouling qualities and structural integrity respectively, - 23 - it is possible to minimise the cost of producing the coatings. Use of separate anti-fouling andstructural monomers enables coatings with the desired properties to be produced using the minimum amount of fluorinated alkoxysilanes, hence minimising costs.

A flow diagram showing the manufacture of a sol-gel coating using a polymeric sol-gel process is shown in Figure 3. The manufacturing and application method will now be further described with reference to Figure 3.

The first stage in manufacturing the coating is the preparation of a solvent, indicated by boxes 40, 42 and 44. Typically the solvent is a combination of alcohol and water. The alcohol and water are mixed for around 15 minutes.

The second stage in the process is hydrolysis of the precursor materials. The rate of hydrolysis for each type of precursor monomer is different. In order to achieve a homogeneous network, it is necessary to initiate hydrolysis of the precursor monomers separately and then combine them together. It is important that the various monomers are not allowed to substantially polymerise before they are combined, otherwise a reduction in homogeneity of the network will result.

During stage 2, the network monomers are added to the solvent and a small quantity of acid is added to initiate hydrolysis 46. The acid is preferably a mineral acid.

- 24 - The solution is then mixed for around 30 minutes. As this process takes place, the anti-fouling monomers are added to a separate stock of solvent, and a small quantity of acid is added and the solution mixed for around 30 minutes, box 48. If separate anti-fouling monomers and structural monomers are used, these are also hydrolysed separately, 50. Stage two continues until hydrolysis of each species is significantly underway.

Stage 3, indicated by box 52, involves combining and mixing the hydrolysed network monomers, the hydrolysed anti-fouling and any hydrolysed structural monomers to form a sol. It is important that the network, structural and anti-fouling monomers are mixed when hydrolysis of each type of monomer is significantly underway, and before the onset of polymerisation. This ensures that a homogeneous network is formed. Preferably the hydrolysed monomers are mixed for around thirty minutes. During this time, the hydrolysed monomers polymerise to form a network consisting primarily of siloxane linkages (Si-O-Si), but also containing silicon atoms bonded to fluorinated alkyl chains derived from the antifouling monomers, and silicon atoms bonded to non-hydrolysable groups derived from the structural monomers. These groups are evenly distributed throughout the network.

Preferably, the sol is aged before being applied to the heat exchanger so that the homogenous inorganic network may develop further. Ageing the sol consists of heating the sol over an extended period of time. After the sol - 25 - is aged, a small quantity of distilled water is added and the sol is mixed for a second period.

Stage 4 is indicated by box 54, and consists of depositing the sol onto a surface of a heat exchanger to form a coating. The coating can be applied to the heat exchanger surface by dip coating in air. This coating method involves placing the surface to be coated into a tank of sol. The sol is drained from the tank at a particular rate. The coating thickness is primarily determined by the rate of draining.

After the tank is fully drained, the surface is removed from the tank and left under ambient conditions, preferably for a minimum of 10 minutes.

An alternative form of the dip method may be used, in which the surface is immersed in a tank of sol, and subsequently withdrawn from the tank at a predetermined rate. The thickness of the coating is controlled by the viscosity of the sol and the rate of withdrawal from the tank.

Alternatively, the coating can be applied using a centrifugal technique. This involves placing the surface in a basket and adding sufficient sol to cover the surface. The sol is then emptied from the basket, and the basket is spun to remove excess sol from the surface. The thickness of the coating is controlled by the viscosity of the sol and the rate and duration of spinning.

- 26 - Further coating techniques may be used, including flow (or curtain) coating where the surface is exposed to a flow of sol, and spray coating, where the sol is sprayed directly onto the surface.

After the coating is applied to the surface, the coating is cured at a predetermined temperature for a predetermined period of time to form a hard solid layer on the surface. This is stage 6 in the method of manufacturing and applying the coating, and is indicated by box 56.

The coating is preferably cured for a relatively short period of time at or above the maximum temperature of the gas in the heat exchanger during operation. In the case of a combined cooler for an internal combustion engine, the coating is preferably cured at a temperature of around 250 C for about 10 minutes.

Examples

A number of coatings having different compositions were prepared and tested. The compositions of these coatings is summarised in table 1 below: - 27 -

Coating Brief description of the coating

Coating 1 Coating prepared from TEOS network monomers and MPTMA structural monomers. No anti-fouling monomer was used.

A corrosion resistant coating of approximately 1 pm thickness was produced. The coating had no further functionality.

Coating 2 Replaced the MPTMA of coating 1 with structural monomers having an alkane chain. This changed the organic part of the structural monomer from double to single bonded.

Double bonds are sites where cross linking can occur.

Replacing double bonds with single bonds reduces cross- linking.

Coating 3 In addition to the network and structural monomers, this coating was prepared using aliphatic urethane acrylate organic monomers which polymerised to form an organic polymer network. This allowed a thicker coating, approximately 4-5 pm, to be produced.

Coating 4 Coating prepared similarly to Coating 1, but in which 3% of the MPTMA was replaced with a fluorinated alkoxysilane which lowered the surface energy of the resultant coating. The lower surface energy reduced the ability of other molecules to bond to the coating.

Coating 5 Similar to Coating 4, but with 10% of the MPTMA replaced by a fluorinated alkoxysilane.

Coating 6 Coating prepared similarly to Coating 1 but in which a fraction of the MPTMA was replaced with the polar component TWEEN 80. TWEEN 80 is known to promote a hydrophilic surface, which will affect the morphology of incident water-borne contamination.

Table 1: brief description of the various coatings prepared and tested.

- 28 - Coating 1 A sol was prepared as follows: Hydrolysis of the network monomers: 360.0g TEOS was placed in a beaker, and a mixture of 317.9g industrial methylated spirits (IMS) and 62.3g water acidified with 0.3g hydrochloric acid (HC1) was added thereto.

Hydrolysis of the structural monomers: 140.Og MPTMA was placed in a beaker, and a mixture of 103.7g industrial methylated spirits and 15.2g water acidified with 0.2g HC1 was added thereto.

No anti-fouling monomers were used.

The hydrolysed network monomers and hydrolysed structural monomers were stirred, separately, in sealed beakers for about 30 minutes, after which they were combined, and stirred for about 30 minutes, again in a sealed beaker.

The resulting sol was aged by heating at 80 C for about 24 hours to promote the formation of the inorganic network.

77.4g of water was added to the resulting material.

Coating 2 Hydrolysis of the network monomers: 340.Og TEOS was placed in a beaker, and a mixture of 300.2g industrial methylated spirits and 58.8g water acidified with 0.3g HC1 was added thereto.

Hydrolysis of the structural monomers: 90.8g trimethoxypropylsilane (TMPS) was placed in a beaker, and - 29 - a mixture of 100.9g industrial methylated spirits and 14.8g water acidified with 0.2g HC1 was added thereto.

No anti-fouling monomers were used.

73.5g of water was added to the resulting material.

Coating 3 Hydrolysis of the network monomers: 275.4g TEOS was placed in a beaker, and a mixture of 242.9g industrial methylated spirits and 47.5g water acidified with 0.5g HC1 was added thereto.

Hydrolysis of the structural monomers: l11.5g MPTMA was placed in a beaker, and a mixture of 81.5g industrial methylated spirits and 12.Og water acidified with 0.2g HC1 was added thereto.

No anti-fouling monomers were used.

- 30 - The resulting sol was aged by heating at 80 C for about 24 hours to promote the formation of the inorganic network.

59.5g of water was then added to the resulting material.

The resulting sal was mixed with 1l.36g of UV-curable aliphatic urethane acrylate monomer sold by Akcros Chemicals under product code 210TP30.

Coatings 4 & 5 These coatings are based on coating 1, but with varying fractions of the MPTMA being substituted with an anti- fouling monomer: perfluorodecyltriethoxysilane (PFDTES), a fluorinated alkoxysilane.

Coating 4 Hydrolysis of the network monomers: 180.4g TEOS was placed in a beaker, and a mixture of 159.Og industrial methylated spirits and 31.4g water acidified with 0.3g HC1 was added thereto.

Hydrolysis of the structural monomers: 69.9g MPTMA was placed in a beaker, and a mixture of 51.8g industrial methylated spirits and 7.6g water was added thereto.

Hydrolysis of the anti-fouling monomers: 8.5g PFDTES was placed in a beaker, and a mixture of 2.68g industrial methylated spirits and 0.4g water acidified with 0.2g HC1 was added thereto.

The hydrolysed network monomers, the hydrolysed structural monomers and the hydrolysed anti-fouling monomers were - 31 - stirred, separately, in sealed beakers for about 30 minutes, after which they were combined, and stirred for about 30 minutes, again in a sealed beaker. The resulting sol was aged by heating at 80 C for about 24 hours to promote the formation of the inorganic network. 39.2g of water was then added to the resulting material.

Coating 5 Hydrolysis of the network monomers: 173.8g TEOS was placed in a beaker, and a mixture of l53.Bg industrial methylated spirits and 30.Og water was added thereto acidified with 0.3g HC1.

Hydrolysis of the structural monomers:: 48.5g MPTMA was placed in a beaker, and a mixture of 35.8g industrial methylated spirits and 5.4g water acidified with 0.2g HC1 was added thereto.

Hydrolysis of the anti-fouling monomers: ll.5g PFDTES was placed in a beaker, and a mixture of 3.9g industrial methylated spirits and 0.6g water acidified with 0.05g HC1 was added thereto.

The hydrolysed network monomers, the hydrolysed structural monomers and the hydrolysed anti-fouling monomers were stirred, separately, in sealed beakers for about 30 minutes, after which they were combined, and stirred for about 30 minutes, again in a sealed beaker. The resulting sol was aged by heating at 80 C for about 24 hours to promote the formation of the inorganic network. 35.6g of - 32 - water was then added to the resulting material.

Coating 6 The method of Example 1 was repeated substituting a fraction of the MPTMA with TWEEN 80, a hydrophilic centre.

Hydrolysis of the network monomers: 200.2g TEOS was placed in a beaker, and a mixture of 176.8g industrial methylated spirits and 34.8g water acidified with 0.3g HC1 was added thereto.

Hydrolysis of the structural monomers: 60.7g MPTMA was placed in a beaker, and a mixture of 44.8g industrial methylated spirits and 6.8g water acidified with 0.2g HC1 was added thereto.

No anti-fouling monomers were used.

5.4g TWEEN 80 was added with a further addition of 41.3g of water, the resulting mixture was stirred for a further 1 hour.

The above coating compositions were dip coated onto 50mm square plaques of aluminium, taken from the external tube wall of a heat exchanger. The coatings were cured in air - 33 - at 200 C for 10 minutes and allowed to cool. The plaques were placed in a blown soot stream with a recorded temperature of 250 C. Uncoated plaques were also tested as a reference. Rates of soot accumulation were measured for each plaque and the coatings were inspected for degradation.

The best performing coatings were coating 2 and coating 4.

Six further plaques were prepared: two plaques having coating 2, two plaques having coating 4 and two uncoated plaques as references. The six plaques were exposed to a blown soot stream at 150 C. Rates of soot accumulation were measured. After accumulation of a suitable thickness of soot the plaques were blown in a higher speed air stream, simulating the high service condition of a combined EGR and charge air cooler. Soot was visibly delaminated from plaques having coating 4. This coating had a similar composition to coating 1, but with 3% of the MPTMA replaced by the fluorinated alkoxysilane PFDTES.

The results of these trials are illustrated in Figures 4 and 5. Figure 4 is a plot of cumulative weight loss of the plaque versus time for the period that the plaques were exposed to the gas flow. Steeper gradients correspond to more rapid soot breakaway. The most rapid soot breakaway was for plaques having the fluorinated coating - coating 4.

Table 2 shows the total percentage weight loss for each of the samples tested, and an average weight loss for each of - 34 - the two coatings and for the uncoated samples. The data in Table 2 is plotted in Figure 5. It is clear that there is little difference in soot breakaway between the uncoated plaques and the plaques having coating 2. The samples having the fluorinated coating (coating 4) showed a significantly greater percentage weight loss, indicating a greater tendency towards soot breakaway.

Sample Weight Loss ____________ (%) (avg%) Uncoated 35.0 Uncoated 36.4 35.7 Coating 2 27.8 Coating 2 36.8 32.3 Coating 4 57.7 Coating 4 40.0 48.8 Table 2: percentage weight loss for the plaques exposed to the airstream, and average percentage weight loss for each type of coating.

Further soot removal tests were performed. The tests consisted of wiping the soot covered surfaces of the plaques with a sheet of laboratory paper having a contact area of l4xl4mrn and a weight of 9.515g. A force of 48. 5N was applied to the paper. The plaques were weighed before and after the wipe process, and the weight difference was used to calculate a percentage removal of soot. A mean average percentage soot removal was calculated for each coating type, and the results are illustrated in Table 3.

- 35 - Coating Mean average % soot removal Uncoated 75% Coating 2 90% Coating 4 98% Table 3: percentage soot removal for various coatings using a wipe test.

Soot was removed almost entirely from plaques having coating 4 while both the plaques having coating 2, and the uncoated plaques, retained a soot film.

From Table 1, it is clear that the coated samples enabled a much greater amount of soot to be removed using the same force. The fluorinated coating gave the greatest percentage soot removal.

All the soot covered aluminium samples except for those having coating 4 showed a residual darkened stain, which was difficult to remove.

The tests proved that the most suitable anti-fouling coating for a combined EGR and charge air cooler is coating 4. Coating 4 was made from 180.4g TEaS, 69.9g NPTMA, 8.5g PFDTES. The molar masses of these three compounds are 208.3g, 248.4g and 610.4g respectively. The molar ratio TEOS:MPTMA:PFTDES is therefore approximately 75:24:1. Therefore, this coating is produced from a solution of hydrolysed monomers wherein 75% of the - 36 - monomers are network monomers (TEDS), 24 % are structural monomers (NPTMA) and 1% are fluorinated monomers (PFTDES) In this coating, substantially 75% of the silicon atoms are derived from the network monomers, substantially 24% are derived from the structural monomers, and substantially 1% are derived from the anti-fouling monomers. Therefore, 24 % of the silicon atoms are bonded to a non-hydrolysable organic moiety derived from the structural monomers, and 1 % of the silicon atoms are bonded to a fluorinated alkyl chain derived from the anti- fouling monomers.

Further tests proved that effective coatings may be produced from a solution of hydrolysed monomers, wherein upto 30% of the monomers are structural monomers and upto 2% are anti-fouling monomers. Such coatings would have up to 30% of silicon atoms in the network being derived from structural monomers and hence being bonded to a non- hydrolysed group, and up to 2% of silicon atoms being derived from anti- fouling monomers, and hence being bonded to a fluorinated alkyl group.

Further tests showed that if PFOTES (1H,lH,2H,2HperfluorooctyltriethOXYsilane) is used instead of PFTDES in coating 4, the same levels of performance may be achieved. PFOTES is preferable as it is relatively cheap and can be purchased in bulk, whereas PFDTES is much more expensive, and generally only available in small quantities.

- 37 - It should be understood that the invention has been described by way of example only and that modifications in detail may be made without departing from the scope of the invention as defined in the claims.

Claims

- 38 - Claims 1. A heat exchanger for use in an exhaust gas recirculation

system of a vehicle, the heat exchanger having a coating on at least part of a surface, the coating comprising of a siloxane based network, the network comprising silicon atoms, oxygen atoms and fluorinated alkyl groups, with each silicon atom being coupled to another silicon atom through siloxane linkages, wherein a plurality of the silicon atoms are also bonded to a fluorinated alkyl group, the fluorinated alkyl groups being distributed substantially evenly throughout the bulk of the network.
2. A heat exchanger as claimed in Claim 1, wherein the coating further comprises organic moieties bonded to a plurality of the silicon atoms, the organic moieties being distributed substantially evenly throughout the bulk of the network.
3. A heat exchanger as claimed in Claim 2, wherein 2 to 30% of the silicon atoms in the network are bonded to an organic moiety, and 0.1 to 2% of the silicon atoms are bonded to a fluorinated alkyl group.
4. A heat exchanger as claimed in Claim 2 or Claim 3, wherein substantially 24% of the silicon atoms in the network are bonded to an organic moiety, and substantially 1% of the silicon atoms are bonded to a fluorinated alkyl group.

- 39 -
5. A heat exchanger as claimed in any one of Claims 2 to 4, wherein the organic moieties are methacrylate groups.
6. A heat exchanger as claimed in any preceding claim, wherein the fluorinated alkyl groups have a carbon chain length of 2 to 12 carbon atoms.
7. A heat exchanger as claimed in any preceding claim, wherein the thickness of the coating is in the range 0.5 to 2 pm over the greater part of its area.
8. A heat exchanger as claimed in any preceding claim, wherein the coating is capable of durable operation at temperatures up to 250 C.
9. A heat exchanger as claimed in any preceding claim, wherein the surface is made from aluminium or an aluminium alloy.
10. A heat exchanger as claimed in any preceding claim, wherein the surface is a surface of a plastic end cap or plastic header unit of the heat exchanger.
11. A heat exchanger as claimed in any preceding claim, wherein the surface is an internal surface of the heat exchanger.
12. A heat exchanger as claimed in any one of Claims 1 to 10, wherein the surface is an external surface of the heat exchanger.

- 40 -
13. A heat exchanger as claimed in any preceding claim, for use in a combustion product circuit of a vehicle having an internal combustion engine.
14. A heat exchanger as claimed in any preceding claim, wherein the heat exchanger is a combined exhaust gas recirculation (EGR) and charge air cooler.
15. A vehicle having an internal combustion engine and a combined EGR and charge air cooler as claimed in Claim 14.
16. A method of coating a surface of a heat exchanger with an antifouling coating, the method comprising the steps of: hydrolysing network monomers to form hydrolysed network monomers; hydrolysing anti-fouling monomers to form hydrolysed anti-fouling monomers; combining and mixing at least the hydrolysed network monomers and the hydrolysed anti-fouling monomers to form a solution of hydrolysed monomers; allowing the hydrolysed monomers to polymerize to form a sol; applying the sol to a surface of a heat exchanger; curing the sol to form a hard coating on the surface of the heat exchanger; wherein the network monomers have the general formula: SR'aR2b (OR3) c [1] - 41 - wherein R' and R2 are organic moieties, R3 is an organic moiety, a and b are integers in the range 0 to 2, and c is an integer in the range 2 to 4, such that (a+b+c) equals 4; wherein the anti-fouling monomers have the general formula: SjR4cLR5e(OR6Yf [2] wherein P.4 is a fluorinated alkyl group, P.5 and R6 are organic moieties, d is an integer in the range 1 to 3, e is an integer in the range 0 to 2, and f is an integer in the range 1 to 3, such that (d+e+f) equals 4.
17. A method as claimed in Claim 16, further comprising the step of: hydrolysing structural monomers to form hydrolysed structural monomers; and wherein the step of combining and mixing at least the hydrolysed network monomers and the hydrolysed anti- fouling monomers further includes combining and mixing the hydrolysed structural monomers to form the solution of hydrolysed monomers; wherein the structural monomers have the general formula: SiRgR8h(OR9) k [3] wherein R7 is a non-hydrolySable group, R8 and R9 are - 42 - organic moieties, g is an integer in the range 1 to 3, h is an integer in the range 0 to 2, and k is an integer in the range 1 to 3, such that (g+h+ k) equals 4, and wherein the structural monomers are not the same species as the network monomers.
18. A method as claimed in Claim 17, wherein the step of combining and mixing the hydrolysed network monomers, the hydrolysed anti-fouling monomers and the hydrolysed structural monomers takes place before substantial polymerization of the hydrolysed monomers can occur.
19. A method as claimed in Claim 17 or Claim 18, wherein 2 to 30% of the monomers comprising the solution of hydrolysed monomers are hydrolysed structural monomers, and 0.1 to 2% of the monomers are hydrolysed anti-fouling monomers.
20. A method as claimed in any one of Claims 17 to 19, wherein the molar ratio of network monomers: structural monomers: anti-fouling monomers is substantially 75:24:1.
21. A method as claimed in any one of Claims 17 to 20, wherein the network monomers are tetraethoxysilafle (TEOS)
22. A method as claimed in any one of Claims 16 to 21, wherein the anti-fouling monomers are 1H,lH,2H,2Hperfluorooctyltrieth0XySil (PFOTES).
23. A method as claimed in any one of Claims 17 to 22, - 43 - wherein the structural monomers are 3- (trimethoxysilyl) propylmethacrylate (MPTMA)
24. A method as claimed in any one of Claims 16 to 23, wherein sol is aged prior to the sol being applied to the surface, the ageing being achieved by heating the sol at a predetermined ageing temperature for a predetermined ageing time.
25. A method as claimed in Claim 24, wherein the ageing temperature is in the range 60 to 90 C, and the ageing time is in the range 1 - 48 hours.
26. A method as claimed in any one of Claims 16 to 25, wherein the sol is applied to the surface by immersing the surface in a tank of sol and draining the sol from the tank at a predetermined rate.
27. A method as claimed in any one of Claims 16 to 25, wherein the sol is applied to the surface of the heat exchanger by immersing said surface in a tank containing the sol and subsequently withdrawing the surface from the tank at a predetermined withdrawal rate.
28. A method as claimed in any one of Claims 16 to 25, wherein the sol is applied to the surface by covering the surface with sol and subsequently spinning the surface at a predetermined rate of rotation to remove excess coating.
29. A method as claimed in any one of Claims 16 to 25, - 44 - wherein the sol is applied to the surface by exposing the surface to a flow of sol.
30. A method as claimed in any one of Claims 16 to 29, further comprising the step of assembling the heat exchanger from its component parts prior to the coating being applied, and wherein the step of applying the sol to the surface involves filling the inside of the heat exchanger with sol and allowing excess sol to drain, such that a plurality of internal surfaces of the heat exchanger are coated.
31. A method as claimed in any one of Claims 16 to 29, further comprising the step of assembling the heat exchanger from its component parts prior to the coating being applied, and wherein the step of applying the sol to a surface involves immersing the entire heat exchanger into a tank containing the sol and subsequently withdrawing the heat exchanger from the tank at a predetermined withdrawal rate such that all the internal and external surfaces of the heat exchanger are coated.
32. A method as claimed in any one of Claims 16 to 31, wherein the coating is cured at a temperature in the range of 150 to 250 C for between 5 and 30 minutes.
33. A method as claimed in any one of Claims 16 to 32, wherein the heat exchanger is a combined exhaust gas recirculatiofl (EGR) and charge air cooler.

- 45 -
34. A heat exchanger having an anti-fouling coating on at least part of a surface, the surface being coated by a method as claimed in any one of Claims 16 to 33.
35. A heat exchanger as claimed in Claim 34, wherein the surface is an internal surface of the heat exchanger.
36. A heat exchanger as claimed in Claim 34 or Claim 35, wherein the surface will be exposed in use to combustion products.
37. A heat exchanger as claimed in any one of Claims 34 to 36, wherein the average thickness of the coating is in the range 0.5 to 2 pm over the greater part of its area.
38. A heat exchanger as claimed in any one of Claims 34 to 37, the heat exchanger further including a shell, wherein the surface is a surface of the shell.
39. A heat exchanger as claimed in any one of Claims 34 to 38, the heat exchanger further including a plurality of heat exchange tubes, wherein the surface is a surface of the heat exchange tubes.
40. A heat exchanger as claimed in any one of Claims 34 to 39, wherein the heat exchanger further includes a header unit, and the surface is a surface of the header unit.
41. A heat exchanger as claimed in any one of Claims 34 - 46 - to 40, wherein the heat exchanger further includes end caps, and the surface is a surface of the end caps.
42. A heat exchanger as claimed in any one of Claims 34 to 41, wherein the surface is made of aluminium or an aluminium alloy.
43. A heat exchanger as claimed in any one of Claims 34 to 42, wherein the surface is made of a plastics material.
44. A heat exchanger as claimed in any one of Claims 34 to 43, wherein the heat exchanger also has a coating on at least part of an external surface, the external surface being coated by a method as claimed in any one of Claims 16 to 33.
45. A heat exchanger as claimed in any one of Claims 34 to 44, wherein the coating is capable of durable operation at temperatures up to 250 C.
46. A heat exchanger as claimed in any one of Claims 34 to 45, the heat exchanger being for use in a combustion product circuit of a vehicle having an internal combustion engine.
47. A heat exchanger as claimed in Claim 46, wherein the heat exchanger is a combined exhaust gas recirculation (EGR) and charge air cooler.
48. A vehicle having an internal combustion engine and a - 47 - combined EGR and charge air cooler including a heat exchanger as claimed in Claim 47.
49. A heat exchanger having a coating, the heat exchanger being substantially as herein described with reference to the accompanying drawings.
50. A method of coating one or more internal surfaces of a heat exchanger substantially as herein described with reference to Figure 3 of the drawings.