WO2015189609A1

WO2015189609A1 - Magnet coating

Info

Publication number: WO2015189609A1
Application number: PCT/GB2015/051700
Authority: WO
Inventors: Owen Morris; James Cooke
Original assignee: Metal Improvement Company Llc T/A Everlube Products Uk
Priority date: 2014-06-11
Filing date: 2015-06-10
Publication date: 2015-12-17
Also published as: GB201410440D0

Abstract

Disclosed is a composition for coating a magnet. The composition comprises phenolic resin, epoxy resin and metal flake. The epoxy resin is present in the composition in an amount greater than the phenolic resin (by weight percent of total composition). Also disclosed are magnets coated with the coatings and methods for applying the magnet coatings.

Description

MAGNET COATING

FIELD OF THE INVENTION

The invention relates to magnet coatings, magnets coated with the coatings and methods for applying the magnet coatings.

BACKGROUND

Magnets and magnet systems are used in a wide range of applications, for example in engine motors and fridges. It is known that under certain conditions magnets can corrode. Naturally, this is undesirable because it means that the magnetic properties diminish. Rare earth magnets are especially known to be susceptible to corrosion. For example, Nd-based magnets are considered to corrode by oxidation of the Nd and will typically crumble apart when corrosion sets in. Therefore the magnetic function is lost (Am. J. Orthod. Dentofacial Orthop., 2006, Sep;130(3):275.e11-5). Coating compositions for application to magnets to inhibit corrosion are therefore desirable. In particular, coating compositions for magnets that can be used in high heat settings are particularly desirable because the corrosion tends to occur faster at higher temperatures.

Coating compositions for high heat applications are known. These coatings are not applied in a magnetic setting, but are more for corrosion resistance of heated steel, e.g. chimney stacks and hot pipework. Such coatings tend to be silicone-based rather than epoxy-based; the inventors are not aware of any such coatings that are epoxy-based. As an example, an aluminium coating named DC6571-805 (available from Indestructible Paint Ltd.) is able to withstand temperatures of up to 1500 °F (816 °C) when properly applied. This coating requires metal-to-metal contact between a metallic substrate and the aluminium

pigmentation for proper adhesion. It has high heat resistance, an aluminium pigmentation and good durability, with applications in, for example, the aerospace field, boilers, engines and exhausts. These high temperature coatings are unsuitable for application to magnets because the use of silicone binders makes it very difficult to glue the magnet to another magnet or to another structure once coated.

CN103059692 describes an epoxy powder coating for coating a magnetic ring. The epoxy powder coatings comprise 40 to 70 parts by weight of epoxy resin, 10 to 50 parts by weight of phenolic resin curing agent, 0.1 to 2 parts by weight of curing accelerator, 2.5 to 15 parts by weight of flame retardant synergist, 30 to 60 parts by weight of pigment and filler, and 0.3 to 2 parts by weight of assistant. These compositions have highly brominated epoxy resins for flame retardation. EP2653302 describes a transition metal-Boron magnet material comprising a transition metal-Boron magnet powder and a coating comprising, in an amount by weight of the magnet powder, 0.1-1 wt% of an organotitanate or organozirconate coupling agent, an epoxy bonder, a curing agent, an accelerator and a lubricant.

CN 102653643 A describes a composite coating for improving corrosion resistance of a Nd- Fe-B magnet. The composite coating has a zinc-based coating, and a reinforced protective coating. The zinc-based coating consists of, by mass percent, 65%-95% of flake zinc powder, 1 %-30% of flake aluminium powder and 0%-5% of an amorphous composite chromic salt compound. The reinforced protective coating is one of a wear-resisting organic silicone resin coating, a polytetrafluoroethylene coating and an epoxy resin coating.

Chromium compounds show high toxicity, which is less desirable.

Anti-corrosion electrophoretic coatings are also known. For example, a flexible epoxy polymer coating process for heat transfer coils that is said to impart resistance to under-film creep, rust and blisters is described by the Luvata company.

Epoxy resin-based anti-corrosion coatings with metal flakes are known. For example, the Yachtpaint division of Akzo Nobel provides a high solids epoxy primer which contains aluminium flake for use on substrates including prepared fiberglass, steel and aluminium to inhibit corrosion. This is a two-component air-curing coating.

Furthermore, GB2480515 describes a lacquer capable of functioning as an adhesive and corrosion-protective lacquer for rare earth permanent magnets comprising an epoxy resin mixture comprising 1-9 wt% solid epoxy resin(s) with an epoxide number of up to 2 Eq/kg, 1- 50 wt% multifunctional solid epoxy resin(s) with an epoxide number of >4 Eq/kg and a setting agent comprising 5-40 wt% phenol and/or cresol novolac with a melting point >30°C, a setting accelerator, a silane-based epoxy functional adhesion promoter and at least one solvent wherein 5-20 wt% of the epoxy resin mixture includes a viscous epoxy resin based on bisphenol A with an elastomer content of >30 wt% and having a viscosity >5000 mPa.s at 23°C.

Corrosion of Nd-Fe-B permanent magnets has been studied by Drak and Dobrzahski (Journal of Achievements in Materials and Manufacturing Engineering, 2007). They concluded that bonded magnets with a protective lacquer or metal were not as good as polymer-based coatings in terms of their anti-corrosive properties. Magnets can alternatively or additionally be protected from corrosion using a nickel-copper- nickel plating process as described in, for example, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 20, Issues 102, January-February 2007.

CN 102031052 describes a coating that is applied to a metal surface which has been pre- plated onto a magnet. The coating has a blend of one or more kinds of solid powder such as aluminium powder, zinc powder, silver powder, M0S2 powder, WS2 powder and nickel powder, and a binder selected from epoxy resin, phenolic resin, hydrated sodium silicate, silica sol and polyacrylamide.

Spray-applied coatings such as the Everlube 6155 © formulation available from Everlube Products, are currently used as coatings for rare-earth magnets. These coatings are based on phenolic resins, but also show good anti-corrosion properties.

There is a need to provide improved anti-corrosive compositions to apply to magnets, especially for high heat applications such as in motors. It is also important to consider the adhesive properties of the compositions, to maintain the structural integrity of the final article. This is because magnets are often glued together in use and therefore, the better the adhesion, the better the structural integrity of the article (e.g. engine or turbine component).

SUMMARY OF THE INVENTION

As can be seen from the above discussion, many types of coating (not only for magnets) are known. The present inventors have found that it is useful to be able to include metal flakes in polymer coating compositions for magnets, for example to improve anti-corrosion properties and for improving the appearance of the coated articles. The present invention relates to a polymer composition that contains a high amount of epoxy resin compared to phenolic resin which is surprisingly able to hold metal flakes in place better than a polymer composition that is high in phenolic resin compared to epoxy resin. In particular, such compositions when applied to magnets show excellent anti-corrosion properties, good adherence and can be used under high temperature conditions.

The present invention has been devised in order to address at least one of the above problems. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems. In one aspect, there is provided a magnet coating composition, having a phenolic resin component, an epoxy resin component, and metal flake, wherein the epoxy resin component is present in the composition in an amount greater than the amount of the phenolic resin component (by weight percent of the overall composition).

Without wishing to be bound by theory, the inventors consider that known compositions, which were high in phenolic resin content, were able to hold metal flakes in place (when cured). In this way, they formed a barrier to inhibit corrosion. Such formulations sometimes include a small amount of epoxy resin to provide a little additional adhesion. Surprisingly, compositions of the present invention that are rich in epoxy resins compared to phenolic resins provide an improved barrier against water and/or salts and show better action as anti- corrosion coatings compared to known compositions. The presence of a small amount of phenolic resin causes hardening of the coating composition through a cross-linking type mechanism.

In some cases, the weight ratio of epoxy resin components to phenolic resin components is greater than 1 , e.g. at least 1.5, at least 2, at least 3 or at least 3.5. The ratio of epoxy resin components to phenolic resin components may be up to 10, up to 8, up to 6, or up to 5 as an upper limit. The ratio of epoxy resin components to phenolic resin components may be between around 1.5 and 0, between around 2 and 6.5, or between around 4 and 5.

Preferably, the ratio of epoxy resin components is between around 4.2 and 4.8, for example, around 4.4 or 4.7.

The epoxy resin component in the compositions of the invention may be present in an amount at least 10 wt.%, at least 15 wt.%, at least 20 wt.%, at least 25 wt.%, or at least 28 wt.% (which weight percent is of the undried composition). The epoxy resin component in the composition may be present in an amount up to 98 wt.%, up to 85 wt.%, up to 60 wt.%, up to 50 wt.%, or up to 35 wt.% (which weight percent is of the undried composition). The epoxy resin component may be present in an amount ranging from between around 10 and 95 wt.%, between around 12 and 80 wt.%, between around 20 and 75 wt.% or between around 25 and 50 wt.%, and is preferably around 30 wt.% (which weight percent is of the undried composition). There may be one or more (such as two or three) types of epoxy resin component in the composition making up the overall epoxy resin component.

Preferably the epoxy resin component is made up of a single type of epoxy resin.

The phenolic resin component in the compositions of the invention may be present in an amount at least 1 wt.%, at least 2 wt.%, at least 3 wt.%, or at least 5 wt.% (which weight percent is of the undried composition). The phenolic resin component in the compositions of the invention may be present in an amount up to 35 wt.%, up to 20 wt.%, up to 12 wt.%, or up to 8 wt.% (which weight percent is of the undried composition). The phenolic resin component in the compositions of the invention may be present in an amount between around 1 and 35 wt.%, between around 2 and 25 wt.%, between around 3 and 15 wt.%, or between around 5 and 12 wt.%, for example around 7 wt.% (which weight percent is of the undried composition). There may be one or more (such as two or three) types of phenolic resin in the composition making up the phenolic resin component. Preferably the phenolic resin component is made up of a single type of phenolic resin.

The magnet coating compositions of the invention have good anti-corrosion properties. In particular, preferably they are capable of resisting failure of a salt spray test according to ASTM B117 for at least 50 hours, for at least 225 hours, preferably for at least 300 hours, at 15 Mm dried film coating thickness. In some cases they are capable of resisting failure of a salt spray test according to ASTM B117 for at least 300 hours, at least 650 hours, preferably for at least 1000 hours, at 25 pm coating thickness. Preferably they are capable of resisting failure of a salt spray test according to ASTM B117 for at least 450 hours, at least 1000 hours, preferably for at least 1600 hours, at 35 pm coating thickness. There is no upper limit on how long the compositions may preferably resist failure of the salt spray test, because a longer resistance time is more desirable. Such resistance levels are readily measurable by the skilled person, and are a useful judge of composition corrosion resistance when applied to magnets, such as rare-earth magnets. In particular, it is preferred for the compositions of the invention to meet or exceed all the aforementioned criteria relating to salt spray testing.

The components of the magnet coating compositions of the invention are mixed together to form a single (i.e. not composite) coating composition for application to magnets. Usually, the magnet coating composition of the invention is homogeneous, and can be

homogeneously applied to a magnet surface.

It is preferable for magnet coating compositions of the invention to have a range of available dry film thicknesses, so that the thickness can be adjusted as desired. In particular, it is preferred that the compositions of the invention have a dry film thickness in a range of between around 1 and 100 pm, between around 5 and 100 pm, or between around 10 and 50 pm. Preferably, the dry film thickness is around 10 pm, around 15 pm, around 25 pm, or around 35 pm. Dry film thicknesses may be measured using, for example, an ISO standard method ISO2808:2007. In general, the magnet coating compositions of the invention are applied to magnets in a single layer, though for larger thicknesses multiple layers may be formed, each layer comprising all the components of the magnet coating composition as set out in the claims. The compositions of the invention may be applied as part of a multi-layer coating with other compositions. In some cases it is preferred that the compositions of the invention are applied in the absence of other layers formed from compositions outside the present invention, although in some cases it may be preferred that the compositions of the invention are applied in addition to other layers formed from compositions outside the present invention.

The metal fiake in the compositions of the invention may impart further benefits because it may be useful for providing pigmentation, and/or additional anti-corrosion properties.

Examples of useful metal flakes include transition metal flake, Group III metals, and include aluminium flake, zinc flake and/or magnesium flake. Especially preferred is aluminium flake. Magnesium flake is most expensive and so is less cost-effective. Preferably the metal flake is formed from a non-toxic metal, i.e. preferably excluding toxic metals such as chromium, lead, arsenic etc. The metal flake is preferably formed from an elemental metal or its oxide rather than a non-oxide compound or an alloy. Preferably the metal flake component is aluminium or aluminium oxide flake and the composition preferably does not contain any other metal flake component.

The metal flake used in the present invention is typically distinct from a metal powder.

Preferably the metal flakes used in the invention are thin, flat sheets of metal, e.g. being significantly larger in two dimensions that in the third dimension. Preferably the

compositions of the present invention do not include metal powder.

The magnet coating composition of the invention may additionally comprise further additives. For example, the magnet coating composition of the invention may optionally further comprise one or more additional different types of resin such as a silicone compound or a polyvinyl resin, a zinc compound, a dispersant, a defoaming agent, a phosphorous containing compound, an accelerator, a wetting agent, a rheology modifier, a surface-active agent (surfactant), an adhesion promotor, and/or a pigment. The further additive(s) may individually be present in an amount of at least 0.1 wt.%, at least 0.3 wt.%, or at least 0.5 wt.% (which weight percent is of the undried composition). The further ingredient(s) may individually be present in an amount of up to 20 wt.%, up to 15 wt.%, or up to 10 wt.% (which weight percent is of undried composition). The further additive(s) may individually be present in an amount between around 0.1 wt.%-20 wt.%, between around 0.2 wt.%-18 wt.%, or between around 0.5 wt.%- 0 wt.% (which weight percent is of undried composition). Preferably, the coating composition comprises one or more or preferably all of zinc phosphite, a phosphoric acid compound, and a silicone compound. Zinc phosphite is an anti-corrosion additive for steel and is used in the present formulations to protect the iron component of the magnet. The phosphoric acid is a catalyst for the curing mechanism, increasing cross-link density which helps with adhesion and chemical resistance. The silicone resin is a surface tension modifier, and helps with the wetting of the substrate, allowing for a more defect-free film, which in some cases may provide a smoother film. A number of different combinations can be used to fulfil these requirements; for example, alternative additives or combinations of additives may be used. For example, zinc phosphate or calcium phosphosilicate might be used in place of or in addition to zinc phosphite. Alternative surface tension modifiers, based on silicone or silicone-free materials, might be used in place of or in addition to the silicone. Nacure XP333® might be used in place of or in addition to phosphoric acid. A combination of these additives may be used.

In a second aspect of the invention, there is provided a use of the magnet coating

composition of the first aspect for reducing or inhibiting the corrosion of a magnet. The magnet coating compositions described above are considered to show excellent anti- corrosion properties, and are expected to find particular utility with permanent magnets. In some examples, the magnet may be a rare-earth magnet. In some examples, the magnet may comprise Nd. In one embodiment, the magnet may comprise an Nd-transistion metal (TM) compound, such as Nd-Fe-B. Rare-earth magnets are considered to be particularly susceptible to corrosion. Therefore, compositions of the invention are expected to find particular application with these types of magnets.

In one embodiment, the compositions of the invention will be particularly useful for high temperature applications. In particular, compositions of the invention are expected to be able to withstand temperatures greater than room temperature, for example, greater than about 70 °C, greater than about 85 °C, greater than about 100 °C, preferably greater than about 150 °C, even more preferably greater than about 200 °C without significant degradation. Preferably, the compositions of the invention are expected to be able to withstand these temperatures for at least 24 hours, such as 36 hours or 48 hours or more, without significant degradation.

In a third aspect of the invention, there is provided a method of protecting a magnet from corrosion, comprising applying a magnet coating composition according to the first aspect onto a magnet or magnet material and subsequently curing the coating composition. The magnet coating compositions disclosed herein advantageously can be applied directly to a magnet surface, for example to the surface of a magnet block, without requiring intervening materials such as metal plating. This is cost-effective and also removes the possibility of any intermediate materials impacting on the level of magnetic flux that can be achieved.

In one embodiment, the method involves applying the magnet coating composition to the magnet material before it is magnetized. This has the advantage that the magnetization will not be affected by later application of a magnet coating composition. For example, neither the composition itself nor the equipment used to apply the composition can affect the magnetization if the coating is applied first. In one embodiment, the coating composition can be applied directly to the surface of a magnet.

In one embodiment the surface of the magnet or unmagnetized magnet material is cleaned and/or primed to remove dirt or grease before the magnet coating composition is applied. Cleaning advantageously allows for a consistent coating of the surface. In a preferred embodiment, the magnet surface may be pretreated with a primer solution, which may be a phosphate primer solution, for example a zinc phosphate primer solution or a manganese phosphate primer solution, prior to application of the magnet coating composition of the first aspect. In some embodiments, the primer solution is applied in a coating layer of approximately 1-5 pm. Advantageously, a judicious selection of primer may provide further anti-corrosion protection.

In a fourth aspect of the invention, there is provided a magnet coated with the magnet coating composition of the first aspect. The magnet coating compositions are expected to find particular utility with permanent magnets. In some examples, the magnet may be a rare-earth magnet. In some examples, the magnet may comprise Nd. In one embodiment, the magnet may comprise an Nd-transistion metal (TM) compound, such as Nd-Fe-B.

Other aspects and embodiments of the invention are described in further detail below.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows three Nd-Fe-B magnets coated with a composition according to the invention at 25 pm dry film thickness (DFT) and subjected to salt spray according to ASTM B-117 after (A) 1000 h; and (B) 1400 h. Fig. 2 shows three Nd-Fe-B magnets coated with a composition according to the invention at 35 Mm DFT and subjected to salt spray according to ASTM B-117 after (A) 1000 h; and (B) 1700 h.

Fig. 3 shows Precision Adhesion Tester (PAT) adhesion tests of (A) electroplated magnets according to the prior art; (B) comparative nickel plated magnets; and (C) magnets coated with a composition according to the invention.

Fig. 4 shows a result of a hardness test using a steel panel coated with a composition according to the invention.

Fig. 5 shows a result of an impact resistance test using a steel panel coated with a composition according to the invention (impact points indicated by arrows).

DETAILED DESCRIPTION; OPTIONS AND PREFERENCES

Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described.

Where values are describes as "at most" or "at least", it is understood that any of these values can be independently combined to produce a range; the upper and lower limits of ranges being intrinsically distinct proposals.

Unless indicated otherwise, the values provided are generally recorded at room temperature; that is, within the range 20-30 °C, for example 20 or 25 °C.

Where non-SI units are provided, it will be understood that these can be converted easily into SI units by the skilled person.

Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above.

Each and every compatible combination of the embodiments described below is explicitly disclosed herein, as if each and every combination was individually and explicitly recited.

Every reference provided herein is hereby incorporated by reference. The term "magnet coating composition" is used herein to describe compositions that can be applied to magnetized materials and materials capable of being magnetized after application of the coating material. It is not intended to be limited to only already-magnetized materials, but also does not encompass materials that must be mixed with other materials in order for magnetization to proceed. Furthermore, the term is used to encompass a composition that has been cured, and a composition that has not been cured (also referred to herein as "undried"), which may contain one or more solvents as discussed below (i.e. an as-prepared blend). The latter encompasses a composition that has one or more solvents used to prepare the mixture (i.e. "undiluted") and compositions that have had one or more further solvents added, in readiness for application to a magnet for example ("diluted").

The term "undried composition" refers to compositions that contain one or more solvent(s). It will be understood by the skilled person that these solvents are not expected to be present (or may be present in negligible quantities) after application and curing of the composition when applied to a magnet or magnet material. The skilled person will also be able to choose one or more particular solvent(s) that are suitable for use in the compositions of the invention. Suitable solvents may include, by way of example and not limitation, aqueous solvents (solvents having water as a primary component), organic solvents (solvents substantially free of water) such as toluene, xylene, alcohols such as methanol, ethanol, propanol, butanol, pentanol, ketones such as propanone, butanone, pentanone and hexanone, PGMA, and acetates such as ethyl acetate, butyl acetate and PM acetate. In the aforementioned, it is contemplated that the solvents may be unsubstituted or substituted with, for example, alkyls such as C1-4 alkyl, aromatic groups such as phenyl, ether, amino, nitro, cyano, carboxy or hydroxyl groups (see also "Foundations of Organic Chemistry", 1993, Oxford University Press). Where appropriate, the solvents may be branched or unbranched (linear).

The term "epoxy resin" refers to synthetic unreacted or reacted resins containing epoxy functional groups (also known as epoxides). Epoxy resins may include, for example, bisphenol A, bisphenol F, Novolac epoxy, aliphatic epoxy and the like. Type 7 epoxies are preferred. These epoxy resins can be of various chain lengths and molecular weight (Mw), for example an Mw between 300-4000, preferably in the range 2500-3000 such as 2700, 2800 or 2900. In some examples the epoxy resin component may be a blend of different epoxy resins. In some examples they may be blended together before being added to the composition. In some examples, they may be blended together in the composition. There may be a number of different types of epoxy resin in the formulation, for example 2 or 3 or 5 different epoxy resins. A suitable epoxy resin may be a polymer of bisphenol A and bisphenol F terminated with an epoxide group, Preferably the epoxy resin is a non- halogenated epoxy resin.

The term "phenolic resin" refers to synthetic reacted or unreacted resins containing phenol functional groups (that is, aromatic 6-membered carbocyclic group with the hydroxyl -OH) functional group. Phenolic resins may include, for example, a resol, a phenol-formaldehyde, a phenolic Novolac, and the like.

The term "silicone" refers to compounds, e.g. resins, having a silicone (polysiloxane) functional group. Polysiloxanes have a chemical formula [R2SiO]_n, wherein R is an organic group and n is an integer (which is usually large, such as 100 or more). The R group may be, in some examples, alkyls such as C1-4 alkyl, aromatic groups such as phenyl, amino, carboxy, or hydroxyl groups (see also "Foundations of Organic Chemistry", 1993, Oxford University Press). Specific silicones may include, for example, those sold under the trade names DC3074 (from Dow Corning ®), and Sil-res ® MSE100 (from Wacker Chemie AG). Sil-res ® MSE100 is a methoxyfunctional methylpolysiloxane (it is a methyl ester of various oligomeric methylsilicates).

The resins referred to herein may have other functional groups, such as alkyl or the like. In some cases, the epoxy resin has phenol functional groups. In some cases, the epoxy resin does not have phenol functional groups. In some cases, the phenolic resin has epoxy functional groups. In some cases, the phenolic resin does not have epoxy functional groups. Preferably, the epoxy resin does not have phenolic functional groups and the phenolic resin does not have epoxy functional groups.

The term "high heat" as used herein is intended to refer to temperatures higher than room temperature, for example, greater than about 70 °C, greater than about 85 °C, and preferably greater than about 100 °C such as 150 °C or 200 °C or more. Compositions suitable for use in high temperature applications should be able to withstand such temperatures without degradation or separation. Preferably, the compositions of the invention are expected to be able to withstand these temperatures for at least 24 hours, such as 36 hours or 48 hours or more, without significant degradation.

As used herein, wt.% of undried composition refers to wt.% of an undiluted composition.

Of particular interest in the present invention are coatings for rare earth magnets, for example magnets which contain Nd. In many cases, Nd magnets also contain a transition metal, such as Fe. Nd-Fe-B magnets are one example of a rare-earth magnet. Dy is another element commonly used in magnets of the type suitable for use with the

compositions of the invention, though it is normally blended with Nd. Permanent magnets, such as Nd-Fe-B magnets, are capable of generating stronger magnetic fields than other sorts of magnet such as ferrite magnets. Rare earth magnets are also particularly vulnerable to corrosion, and so it is of particular interest to provide a good anti-corrosive magnet coating to protect rare earth magnets. In particular, rare-earth magnets are susceptible to attack under conditions of high salt, high acidity and high alkalinity.

Preferably the adhesion of the composition to the magnet body is improved compared to known coatings. Accordingly, it is preferred that the compositions of the invention are applied directly to the magnet body, without intervening layers of material. Of course, the skilled person will understand that the extent of adhesion may vary, for example, with the type of adhesive used and with the conditions under which the adhesion measurements are made. For example, the coating composition of the invention may be able to withstand a separation force of at least 4, at least 6, at least 7, at least 8, or at least 9.5 MPa before adhesion failure in the absence of salt spray. In some cases, the separation forces may exceed 10 MPa before failure in the absence of weathering conditions. In cases where adhesion is important, standard adhesives known in the art may be used to combine, for example, two or more magnets together, or a magnet to another material. The adhesive may be applied to either or both of the magnet and/or the coating. Preferably, for practicality, the adhesive is applied to the coating, as it is more cumbersome to apply the coating to the magnet after the adhesive has been used. Preferably, the bond between the magnet and the coating is able to withstand a separation force of at least 4, at least 6, at least 7, at least 8, or at least 9.5 MPa before adhesion failure in the absence of salt spray. In preferred cases the coated magnets are resistant to both separation of the coating from the magnet surface and to separation of an adhesive from the coating surface under this degree of separation force in the absence of salt spray.

Preferably, the cured compositions of the invention will be resistant to corrosion under a wide range of conditions including exposure to one or more of organic solvents, acidic conditions and alkaline conditions. A composition of the invention may be resistant to corrosion, or corrosion resistant, when two out of three magnets to which the cured composition has been applied showing no rust sports in an ASTM B117 salt spray test after 1000 hours. Preferably, the cured compositions will be resistant for a substantial period of time, for example up to 12 h, up to 24 h, or up to 36 h or more. Preferably, the cured compositions of the invention are substantially non-rigid (i.e. have some flexibility). Preferably, the cured compositions of the invention coated onto a magnet body are able to withstand substantial impact without breaching the coating, for example a 500 g weight dropped from a height of 20 cm.

Preferably, the composition of the invention is a one-pot composition. That is, the composition can be prepared in a single blend for direct application to a magnet. This has advantages over known coating compositions that have two components, because it is more cost-effective and easier for manufacturers and users to employ. In some examples, the compositions are heat-cured (i.e. are cured at a higher temperature than air-cured compositions). Heat curing advantageously provides a shorter curing time and improved chemical resistance over other curing methods.

In one example, the magnet coating composition can be applied using a spray coating method, for example using a spray gun. This method is low-cost, because it does not require the presence of electrodes and the like as is needed for plating or electrodeposition methods, and therefore may also be less energy intensive. Other suitable methods may include dip and dip/spin methods.

Methods of making coating compositions according to the invention will be known to the skilled person. For example the compositions may be prepared using a bladed mixer and stirring the components together in a predefined order. In one example, first the epoxy is dissolved into solvents, then the phenolic resin, silicone resin, any additional solvents and any phosphoric acid are stirred in, then the metal flake (e.g. aluminium flake) and zinc phosphite (where present) are stirred in.

In some examples, coating compositions are applied to a magnet in one or more layers, for example, two or five or more layers.

In some examples, a coating composition of the invention is applied in the absence of other layers formed from compositions outside the present invention. However, in some cases it may be preferred that a composition of the invention is applied in addition to other, further layers formed from a different composition. Such combinations of layers may provide unexpectedly synergistic effects. For example, it may be preferred that a composition of the invention is applied as part of a multilayer coating together with a further layer of a composition having no metal flake component. It may be preferred that a composition of the invention is applied as part of a multilayer coating together with a layer of a composition which is rich in an epoxy resin component. In some examples, a composition of the invention may preferably be applied as part of a multilayer coating together with a layer of a composition which has an epoxy resin component present in an amount (by wt %) greater than an amount of phenolic resin component. Preferably, a composition of the invention is applied as part of a multilayer coating together with a layer of a composition which has no metal flake component (e.g. at least no aluminium flake component) and which has an epoxy resin component present in an amount (by wt %) greater than an amount of phenolic resin component.

In some examples, where the composition is applied as part of a multilayer coating in combination with a further layer as discussed above, the composition of the invention may be applied in one or more layers, for example two or three or five layers. In some examples, the further composition may be applied in one or more layers, for example two or three or five layers. In some cases, the composition of the invention is applied in the same number of layers as the further composition. In some more preferred cases, the composition of the invention is applied in a single layer together with a single layer of the further composition. Preferably, the composition of the invention is applied to the magnet surface and the further composition is subsequently applied on top of the composition of the invention.

In some examples, when it is applied as part of a multilayer coating in combination with a further layer of a different composition as discussed above, the composition of the invention may be applied in a layer thickness greater or less than the further layer. In one example, a layer of a composition of the invention is applied at a thickness of between 10-20 pm. In some cases, the further layer is applied at a thickness of between 10-25 pm. In one example, the layer of composition according to the invention is applied in a thickness of 10 pm and the further layer is applied in a thickness of 15 pm. In one example, the layer of composition according to the invention is applied in a thickness of 10 pm and the further layer is applied in a thickness of 25 pm. In one example, the layer of composition according to the invention is applied in a thickness of 20 pm and the further layer is applied in a thickness of 10 pm.

EXAMPLES

Compositions according to the invention have been prepared and tested for anti-corrosive properties, adhesion, solvent resistance, pencil harness and impact resistance. Details about those experiments and the results are described below. The compositions are one- component heat-curable coatings. The examples are not intended to limit the invention, which is defined by the claims. Primer

Material Percent (wt.) in formulation

Epoxy resin(s) 0.72

Phenolic resin(s) 3.58

Acrylic Resin(s) 4.67

Al flake 0.00

Other Additive(s) 10.42

Solvent(s) 80.62

Composition of the invention

Material Percent (wt.) in formulation

Epoxy resin(s) 29.40

Phenolic resin(s) 6.71

Silicone resin(s) 0.52

Al flake 8.67

Other Additive(s) 5.05

Solvent(s) 49.65

Comparative Example 1

Prochem 66 ® (polyvinylbutyral with anticorrosion additives)

Comparative Example 2

Procoat 100 ® (a migratory self-healing coating)

Comparative Example 3 - a phenolic-based coatinq with small amount of epoxv

Material Percent (wt.) in formulation

Epoxy resin(s) 1.93

Phenolic resin(s) 38.44

Acrylic resin(s) 4.00

Al flake 9.00

Other Additive(s) 5.3

Solvent(s) 41.34

Comparative Example 4 - epoxv-based composition without Al flake

Material Percent (wt.) in formulation

Epoxy resin(s) 30.50

Phenolic resin(s) 6.96

Silicone resin(s) 0.54

Al flake 0.00

Other Additive(s) 10.00

Solvent(s) 51.46

Comparative Example 5

Material Percent (wt.) in formulation

Epoxy resin(s) 0.00

Phenolic resin(s) 35.30

Acrylic resin(s) 4.00

Al flake 0.00

Other Additive(s) 15.60

Solvent(s) 45. 0

Comparative Example 6

Sealer - silicone based clear coating

The composition of the invention has the following properties (of an undiluted coating). An undiluted coating is the composition that is provided, for example, to a customer who then dilutes the coating to a desired viscosity using a solvent. The dilution is commonly 1.2 or 1 :3 (undiluted coating: solvent) by volume, though other dilutions are also contemplated. The components are given in wt% of undiluted composition. Density 0.95 g/cm³

Mass solids 43-48%

Flash point 7 °C

Dry film thickness range 10-50 pm

Operating temperature -73 °C to 200 °C

Theoretical coverage Approx. 7m²/lt @ 25 pm dry film thickness

In each case, the coating was applied using a conventional spray gun having the following properties;

Nozzle sizes 0.8 mm

Air pressure 70-80 Psi

Dilution (solvent: composition) Between around 2: 1 to 3: 1

Dilution solvents MEK, MEK:Toluene, MIBK

Curing:

Flash off: 10 mins @ 90°C

Curing: 60 mins @ 180°C part metal temperature

The following coating method was used:

Coat first side

- Flash off (10 mins @ 90 °C)

Flip the magnets

Coat the second side

- Flash off (10 mins @ 90 °C)

- Full cure (60 mins @ 180 °C)

Anti-corrosion

The corrosion resistance was analysed using ASTM B-1 17 salt spray, performed in triplicate. The magnets were pretreated with a zinc phosphate surface preparation before applying the composition of the invention. A Nd-Fe-B magnet was used in these experiments.

Figs. 1 and 2 show magnets at 25 pm dry film thickness (DFT) after 1000 h and 1400 h salt spray exposure (Fig. 1 ) and magnets at 35 pm after 1000 h and 1700 h salt spray exposure (Fig. 2). As can be seen in Fig. 1 (A) and (B), one rust spot visible on one magnet (see central magnet). This defect was visible after -50 h salt spray suggesting the cause is a defect formed during application. In Fig. 1 (A), it is clear that the other two magnets show no rusting. Fig. (B) shows the same magnets after 1400 h salt spray. Small rust areas (shown by arrows) are just starting to show through the coating, and the coating is now judged to have failed.

Correspondingly, Fig. 2 shows magnets with a 35 μηη coating. In Fig. 2 (A), after 1000 h, there are no visible changes on any of the magnets. In Fig. 2 (B), after 1700 h, blisters were forming on the coating and rust was appearing around the corners.

Further tests were carried out using a composition according to the invention. The results are shown in the table below.

In the table below, where two compositions are given, the first-listed layer was applied to the magnet, and the second-listed was applied subsequently. So, "A + B" means that A was sprayed onto the magnet first and cured/dried, and B was subsequently sprayed on A.

For the results shown in the below table, each coating was applied to three magnets and the failure criterion was the appearance of rust or blisters on two of the three magnets. As a comparison, a NiCuNi plating process achieves approximately 50 hours in ASTM B117.

Formulation Thickness (μιη) Hours to failure

Primer 18 24

Comparative Example 1 15 24

Comparative Example 2 20 72

Comparative Example 3 15 100

25 250

35 400

Comparative Example 4 25 600

Comparative Example 5 25 500

Composition of the invention 10 100

15 300

25 1400

35 1700

Primer + Composition of the invention 18 + 12 250

Composition of the invention + Comparative 15 + 15 200

Example 6

Comparative Example 2 + Comparative 15 + 15 72

Example 6

Composition of the invention + Comparative 10 + 15 1300

Example 4 20 + 10 1500

10 + 25 1350

Comparative Example 1 - Prochem 66 ® (polyvinylbutyral with anticorrosion additives) Comparative Example 2 - Procoat 100 ® (a migratory self-healing coating)

Comparative Example 3 - a phenolic-based coating with small amount of epoxy

Comparative Example 4 - epoxy-based composition without Al flake

Comparative Example 5 - phenolic-based coating

Primer contains anticorrosion additives

It is clear from the above table that compositions according to the invention display improved anti-corrosion properties compared to other formulations. In particular, comparative example 3 has a shorter time to failure (as compared to the composition of the invention) for exactly comparable DFT values. It is 3 times quicker than the composition of the invention to fail at 15 Mm DFT, 5.6 times quicker at 25 pm, and 4.25 times quicker at 35 pm. Other

comparative example compositions fail even more quickly. In the absence of Al flake, the compositions also fail in a shorter time than when Al flake is present as in the compositions of the invention. Adhesion

Fig. 3 shows the PAT adhesion of three magnets coated with a composition according to the invention (C) compared with electrocoated (A) and nickel plated (B) magnets. The electrocoating composition is a commercially available composition based on an epoxy resin. 20 mm dollies were used, and the adhesive used was araldite 2000x and cured for 3 days at ambient temperature. The nickel plating procedure used was as follows:

Step 1 : degrease

Step 2: acid etch

Step 3: nickel or copper strike (if required)

Step 4: nickel plate

The results of the PAT adhesion tests were as follows:

(A) ECoat:

Coating/magnet interface failure; average = 7.09 MPa

Tests 1-3: 6.28 MPa; 6.39 MPa; 8.61 MPa

(B) Nickel plate

Adhesive/nickel interface failure; average = 7.08 MPa

Tests 1-3: 9.12 MPa; 7.59 MPa; 4.52 MPa

(C) Composition of the invention

Adhesive cohesion failure; average = 10.15 MPa

Tests 1-3: 8.15 MPa; 10.82 MPa; 11.49 MPa

Accordingly, use of a coating with a composition according to the invention is limited only by the adhesive cohesion, rather than the interface integrity. On average, the failure was found at higher applied MPa for a composition of the invention (C) compared to the known coatings (A) and (B).

Further tests were performed with Loctite 3430® epoxy adhesive and Loctite 480® cyanoacrylate adhesive. Three different measurements were made, one measurement upon an unweathered coated magnet, one measurement performed where the coated magnet had been exposed to 250 h ASTM B117 salt spray then the dolly had been glued to the magnet, and one measurement where the dolly had been glued to the magnet which was then exposed to 250 h ASTM B117 salt spray. The results of the pull off tests are as follows: Coating Adhesive No weathering 250 h salt spray Dolly adhered to followed by dolly coating followed by adhered to coating 250 h salt spray

Pull off Failure Pull off Failure Pull off Failure

MPa mode MPa mode MPa mode

Composition Loctite 480 ® 15.62 B 20.68 B 2.63 A of the Loctite 3430 ® 4.52 B 6.51 B 6.39 B invention

Nickel plate Loctite 480 ® 5.32 C 3.37 B 0.51 A

Loctite 3430 ® 4.30 B 4.39 B 1.82 B

ECoat Loctite 480 ® 4.11 C 6.02 C

Loctite 3430 ® 6.33 C 4.49 C

Failure modes:

A = cohesive failure of the adhesive

B = failure between the adhesive and the coating interface

C = failure between the coating and the magnet

These results indicate that the Loctite 3430® was a very poor adhesive, and also that the Loctite 480® had very poor salt spray resistance, however it was a very strong adhesive without weathering and showed the adhesion of the composition of the invention to the magnet to be at least 15 MPa, and even gave a result of above 20 MPa after salt spray exposure.

Solvent resistance

These tests were completed by immersing an aluminium panel coated with approx. 25 pm of a composition according to the invention in the test fluid at lab ambient temperature. A visual inspection was completed afterwards to check for blistering and delamination. The coating was unaffected after 24 h immersion in any of the following fluids:

NMP

MEK

Toluene

Distilled water

Mineral spirits

Isopropyl alcohol

10% Hydrochloric acid

10% Sodium hydroxide

Acetone Diethanolamine

Skydrol 500 ®

Trichloroethylene

Hardness

The dried coating of the invention will resist a 9H pencil scratch using a 500 g pressure upon a 45° angle pencil. The photo in Fig. 4 shows such a pencil scratch on a coated steel panel.

Impact resistance

A 500 g ball weight was dropped onto a steel panel coated with a dried coating of the invention from a height of 20 cm. A slight indentation was caused, but no cracking was seen, as shown in the photo of Fig. 5.

Flexibility

A coating according to the invention was applied at a DFT of 25 μιη to a steel panel and bent around a mandrel flexibility cone down to a 4 mm diameter. No cracking was observed.

Summary

According to the above laboratory testing, a composition according to the invention has been proven to give outstanding salt spray resistance when coated over phosphate treated magnets as well as excellent adhesion compared to alternative coatings. The additional mechanical and solvent resistance properties have also proven to be more than adequate for a protective coating.

Claims

1. A magnet coating composition, having a phenolic resin component, an epoxy resin component, and metal flake, wherein the epoxy resin component is present in the composition in an amount greater than the amount of the phenolic resin component by weight percent of the total composition.

2. A magnet coating composition according to claim 1 , wherein the ratio of epoxy resin components to phenolic resin components is between around 1.5-10, optionally between around 4.2-4.8.

3. A magnet coating composition according to any one of the preceding claims, wherein the epoxy resin component is present in an amount at least 10 wt.%, optionally at least 25 wt.% by percent of the undried composition.

4. A magnet coating composition according to any one of the preceding claims, having between 1-35 wt.% of phenolic resin component by percent of the undried composition.

5. A magnet coating composition according to any one of the preceding claims, having between 5-12 wt.% of phenolic resin component by percent of the undried composition.

6. A magnet coating composition according to any one of the preceding claims, which is capable of resisting failure of a salt spray test according to ASTM B117 for at least 150 hours, optionally at least 225 hours, optionally at least 300 hours, at 15 pm coating thickness.

7. A magnet coating composition according to any one of the preceding claims, which is capable of resisting failure of a salt spray test according to ASTM B117 for at least 300 hours, optionally at least 650 hours, optionally at least 1000 hours, at 25 pm coating thickness.

8. A magnet coating composition according to any one of the preceding claims, which is capable of resisting failure of a salt spray test according to ASTM B117 for at least 450 hours, optionally at least 1000 hours, optionally at least 1600 hours, at 35 pm coating thickness.

9. A magnet coating composition according to any one of the preceding claims, having a dry film thickness in a range of 5-100 pm, optionally in a range of 10-50 pm.

10. A magnet coating composition according to any one of the preceding claims, wherein the metal flake comprises a transition metal or a group III metal flake, optionally comprising aluminium flake.

1 1. A magnet coating composition according to any one of the preceding claims, further comprising a silicone compound.

12. Use of a magnet coating composition of any one of the preceding claims for reducing or inhibiting the corrosion of a magnet, optionally a rare-earth magnet, optionally a magnet comprising Nd-Fe-B.

13. Use of a magnet coating composition according to claim 12, for reducing or inhibiting the corrosion of a magnet at a temperature greater than 70 °C.

14. A method of protecting a magnet from corrosion, comprising applying a magnet coating composition according to any one of claims 1-11 onto a magnet or magnet material and subsequently curing the coating composition.

15. A method according to claim 14, comprising providing an unmagnetized magnet material, applying and subsequently curing the magnet coating composition, and

subsequently magnetizing the magnet material to provide a magnet coated with the magnet coating composition.

16. A method according to any one of claims 14- 5, wherein a surface of the magnet or magnet material is cleaned and/or primed before application of the magnet coating composition.

17. A magnet coated with the magnet coating composition of any one of claims 1-11 , which is optionally a rare-earth magnet, optionally a magnet comprising Nd-Fe-B.

18. A magnet coating composition or magnet coated with the magnet coating composition as described herein with reference to the drawings.