Technical field

The present invention relates to the field of seals and bearings. More specifically, the present invention relates to a low friction seal and a method for manufacturing the seal.

Background

Seals are used to prevent leakage between two environments. Seals can be used, for example, to retain a fluid, separate fluids or to prevent the transmission of particulate contaminants from one environment to another.

Seals can be used in static devices and also in non-static devices such as rolling element bearings. An example of a typical elastomer lip seal that could be used in a bearing is shown in Figure 1 . Non-static devices rely on the seals to retain lubricant, prevent water ingress and to prevent particulate, such as grit, contamination of the easily damaged rolling element. However, as will be appreciated, friction during use leads to wear on the seal which will eventually lead to failure of the seal. It is an object of the present invention to provide an improved seal. Such a seal would preferably have a low friction surface and would preferably be hard wearing under normal use conditions. There is a desire for a seal that will overcome, or at least mitigate, some or all of the problems associated with the seals of the prior art or at least a useful or optimized alternative. Summary

In a first aspect, the present invention provides an elastomeric lip seal having a surface coating comprising a polymerised diallylamide or diallylamine.

In a second aspect, the present invention provides a method for manufacturing a coated elastomeric lip seal, the method comprising;

providing an elastomeric lip seal having a contact surface;

providing a coating material on at least the contact surface; and

curing the coating material to provide a coating on the contact surface of the elastomeric lip seal,

wherein the coating material comprises diallylamide or diallylamine.

In a third aspect, the present invention provides an elastomeric lip seal obtainable by the method disclosed herein.

In a fourth aspect, the present invention provides a bearing comprising the elastomeric lip seal described herein. In a fourth aspect, the present invention provides the use of an diallylamide or diallylamine coating to reduce the surface friction and/or increase the wear- resistance of an elastomeric lip seal.

Brief description of the drawings

The present invention will now be described further with reference to the accompanying drawing, provided by way of example, in which:

Figure 1 shows a cross-section of part of a seal according to the invention. Detailed Description

The present invention will now be described further. In the following passages different aspects/embodiments of the invention are defined in more detail. Each aspect/embodiment so defined may be combined with any other

aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Figure 1 shows an example of a shaft seal 10 comprising a metal casing 15 to which an elastomeric sealing lip 20 has been bonded. The seal is mounted in an annular gap between the bore of a bearing housing 30 and a shaft 35, whereby the sealing lip 20 has a contact surface 25 which bears against a counterface on the shaft 35. To ensure that the lip remains in contact with the shaft, the lip is preloaded with a garter spring 40. The contact surface 25 is provided with a wear-resistant coating material. According to the invention, the coating material comprises a polymerised diallylamide or diallylamine. In one embodiment the coating may comprise a diallylamide copolymerised with a diallylamide.

The present inventors have discovered that diallylamine and diallylamide and their quaternary salts can be subjected to fairly rapid cyclopolymerisation to give a robust low friction coating. In particular, the disclosed compounds have been found to bind well to flexible seal material.

The coating composition is especially suitable for use in coating a contact surface of a seal. This is because of the ease of application and the enhanced wear and friction resistant properties that can be obtained. It will be appreciated, however, that the coating may also be applied to other surfaces associated with seals and bearing systems. The coating has been found to bind strongly to metal surfaces, as well as wood, glass, plastics and ceramics, and is therefore suitable for application and use on any contact surface in a bearing. For example, the coating may be applied to a bearing raceway, a rolling element and/or a cage that retains the rolling elements. The composition is especially suitable for use on an elastomeric seal surface since it does not fracture or become detached if the supporting seal surface is flexed. The compounds used in the coating of the present invention are UV-curable. This allows for quick and easy processing of the coating. In addition, the compounds do not require organic solvents in order to be coatable. Accordingly, the manufacturing process may be more environmentally friendly than known coating systems.

The polymer networks formed from the polymerised diallylamides and

diallylannines have been found to have higher surface energies than conventional coatings. Without wishing to be bound by theory, it is believed that this property gives rise to the observed enhanced adhesion and shear strength.

The diallylamide is preferably a polymer of formula I or a quaternary salt thereof:

(I)

wherein n is 1 , 2, 3 or 4,

wherein each Ri is independently selected from H, a C1-C6 alkyl and a

substituted C C ₆ alkyl,

wherein, when n is 2, 3 or 4, R ₂ is a bridging group, and

wherein, when n is 1 , F¾ is an optionally substituted C1-C12 alkyl, or C2-C12 alkenyl or alkynyl.

The diallylamide is preferably a polymer of formula II or a quaternary salt thereof:

(II)

wherein n is 1 , 2, 3 or 4,

wherein each R-i is independently selected from H, a C-i-C ₆ alkyl and a substituted C1-C6 alkyl,

wherein, when n is 2, 3 or 4, R ₂ is a bridging group, and

wherein, when n is 1 , R ₂ is an optionally substituted Ci-C-| ₂ alkyl, or C ₂ -Ci ₂ alkenyl or alkynyl.

By quaternary salt is meant a compound of formula (III) or (IV). That is, the nitrogen of the diallylannide or diallylannine has a formal positive charge to form quaternar salt with a counteranion.

(HI) (IV)

wherein R ₃ is additionally selected from H, a C-i-C ₆ alkyl and a substituted C-i-C ₆ alkyl. The quaternary salts are particularly advantageous because they can be formed into ionic solutions for ease of handling and coating. The quaternary salt will be accompanied with any suitable counterion, such as, for example, CI- or - OH. The PF ₆ - counterion has been found to confer total water impermeability to the final coating, whereas the CI- counterion leads to a hydrophilic coating. BF - provides an intermediate hydrophilicity. It will also be appreciated that any appropriately situated pair of Ri groups in the foregoing structures may form a 5 or six membered cycloalkyl ring together without hindering the polymerisation mechanism and functionality of the compound. When n is 1 , the F¾ group may form a 5 or six membered cycloalkyl ring together with any appropriately situated Ri group. In addition, in the quaternary salts, the R ₃ group may form a 5 or six membered cycloalkyl ring together with any appropriately situated Ri group or the R ₂ group. Advantageously, n is 2, 3 or 4. The selection of molecules having multiple diallyl functionalities increases the degree of polymerisation (DP) that is observed in the final product. As a consequence, strong three-dimensional polymer networks may be formed. Most preferably n is 2. This strikes a balance between the synthetic complexity of generating the polymerisable monomers and the structural benefit of increased DP.

Preferably, R ₂ has from 1 to 18 carbon atoms, and optionally one or more oxygen, silicon, nitrogen or sulphur atoms, which form the bridge. That is, the number of carbon atoms forming the chain between the carbonyl carbons (not included) of each allylamido group or between the nitrogen atom of each allylamino group is from 1 to 18 and optionally further includes one or more additional oxygen, silicon, nitrogen or sulphur atoms. More preferably from 2 to 12 and most preferably from 3 to 6 carbons. This does not preclude the presence of addition carbon atoms, provided these do not form part of the linking chain. The carbons may form a saturated or unsaturated alkyl chain and the chain may further include cyclic groups including aryl groups (only the shortest chain of carbon atoms is considered to form part of the bridge).

The bridge preferably comprises repeating units selected from polyamide, polyurethane, polyether, polytetrafluoroethylene (PTFE) and polyvinyl. More specifically, the preferred bridge group may be: Bridge type Repeating unit (one or more of:)

Polyethylene CH ₂

Polystyrene CH ₂ CH(C6H ₅ ) where the phenyl ring is optionally

substituted

Polyisobutylene CH ₂ CH(CH(CH ₃ ) ₂ )

Polyisoprene CH ₂ CH(CH ₃ )

Polytetrafluoroethylene CH ₂ (CF ₂ ) _X CH ₂

Polyvinyl idenefluoride CH ₂ (CF ₂ CH ₂ ) _X

Polyethyleneoxide (OCH ₂ CH(CH ₃ ))xO

Nylon CH ₂ (NHCOCH ₂ ) _x CH ₂

Peptide CH ₂ (NHCOCHR) _x CH ₂

Polyurethanes -NH-CO-O-

Polyesters -RC(O)OR'- where R and R' are organic groups such as hydrocarbyl

Polysiloxanes e.g. -SiO ₂ -, -R ₂ SiO- or -R ₂ Si ₂ O ₃ - where R is an organic group such as hydrocarbyl

Polyacrylates -CH ₂ C(COOH)H-

Polyureas -NHCONH-

Polythioureas -NH-C(S)-NH-

Where the bridge is substituted, it is preferably substituted with one or more groups selected from C Ci ₂ alkyl, C ₂ -C-| ₂ alkenyl, C ₂ -C-| ₂ alkynyl, aryl, heteroaryl, halo, Ci-Ci ₂ alkoxy, Ci-C-i ₂ amino and Ci-C-i ₂ amido.

In a most preferred embodiment, n is 2 and R ₂ is a C-i-C-is alkyl chain, more particularly a C ₂ -C ₈ chain and most preferably C ₃ to C ₆ . Preferably R ₂ is unsubstituted. Preferably the coating has a mean thickness of from 0.05 to 1 mm. More preferably the coating has a mean thickness of from 0.1 to 0.5mm. Most preferably the coating has a mean thickness of about 0.25mm. The thickness can be measured by techniques known in the art and is measured from the contact surface of the elastomeric lip seal to the upper surface of the coating,

disregarding any surface texturing extending up from or into said upper surface. This provides a sufficient wear surface without detracting from the inherent properties of the material selected for the elastomeric seal.

The coating may further comprise additive compounds. These include well known additives such as dyes. In an advantageous further development, the coating comprises friction modifiers, such as PTFE and/or molybdenum disulphide and/or graphite. Thus, a low-friction and wear-resistant seal is obtained.

In a further advantageous development, the coating comprises fillers or blowing agents for providing a surface texture. Blowing agents are used to produce bubbles in the coating material. The advantages of this surface texturing are discussed below. If the bubble is formed close to the surface then it may be used to provide a void or dimple in the surface. If the bubble is below the surface then it may form a dome on the surface of the coating layer. Blowing agents include physical blowing agents such as CFCs, HCFCs, hydrocarbons (e.g. pentane, isopentane, and cyclopentane) and solid or liquid CO2. The bubble/foam-making process often requires heat initiation.

Chemical blowing agents such as isocyanate and water (which react to release CO2) are especially useful for forming expanded polyurethanes. Other examples include azo-, hydrazine and other nitrogen-based materials and sodium

bicarbonate.

The coating material may comprise particulate material (filler) having a mean longest diameter of from 100 to 2500 nanometers. These particles can contribute to the surface texture. Furthermore, particles can contribute to the wear resistance of the coating. In one embodiment the particulate material is a soluble filler such as sodium chloride crystals. Thus, after the coating layer is formed the particulate material can be washed away to leave voids in the structure. According to a second aspect there is provided a method for manufacturing a coated elastomeric lip seal, the method comprising;

providing an elastomeric lip seal having a contact surface;

providing a coating material on at least the contact surface; and

curing the coating material to provide a coating on the contact surface of the elastomeric lip seal,

wherein the coating material comprises a diallylamide or diallylamine.

Preferably the method described herein is for manufacturing an elastomeric lip seal with a coating as described herein. That is, any feature described with relation to the method or the seal may be applied equally to the other.

Lip seals and configurations of such seals are well known in the art. The seal has a contact surface for contacting, in use, a movable surface. The movable surface (counterface) is the surface against which the seal operates and is not

particularly limited. For example, the counterface may be a rotatable shaft, as shown in Figure 1 , or a surface of the rotatable bearing ring in a rolling element bearing. Depending on the application and strength requirements, the counter- surface may comprise any suitable material. For example, a plastic, a synthetic or a metal substrate may be used.

The elastomeric seal, in particular the seal lip, is preferably formed of a

deformable elastomer. Preferred elastomers for seals include acrylate rubber, fluoro rubber, nitrile rubber, hydrogenated nitrile rubber, or mixtures of two or more thereof. Preferably the contact surface of the elastomeric lip seal is substantially smooth. This allows for easy manufacture of the starting material.

The coating step is preferably carried out with a spray. The compositions described herein may be formed into an oil for solvent-free application, or may be used in an aqueous form for easy handling. Spray application apparatus is well known in the art. Dipping or brush application may also be used in accordance with the method of the invention. Preferably the coating material is cured with UV radiation. The compounds of the present invention are UV curable. The coating may be cured in sunlight or, more preferably with a UV lamp. Low power UVA 320-400nm lamps are suitable. In one embodiment the curing treatment may be selectively applied to the coating material. In this way, uncured material can be removed from the surface to provide a surface texture in a desired distribution on the coated surface.

The coating may further comprise a photopolymerisation initiator. Such

compounds are well known in the art. Exemplary photo-initiators include AIBN and benzoylperoxide.

In a further development, the method comprises an additional step of processing the coating material to provide a textured coating on the contact surface of the elastomeric lip seal. The step of processing the coating material may comprise:

(i) laser etching the coating material; and/or

(ii) stamping a texture onto the coating material; and/or

(iii) initiating a blowing agent included in the coating material.

In seals lubricated with an oil or grease, surface texturing has several benefits. For example, it is thought that a dimpled surface texture on the contact surface enhances the formation of a hydrodynamic oil-film (i.e. increases film thickness), thereby reducing friction. The surface texture may also be adapted to effect a hydrodynamic pumping action in a desired direction. For example, a seal lip may have helical ribs at an air side of the lip, which are designed to pump escaping oil back towards the seal lip. This not only provides the lip with additional lubrication

(thereby reducing friction), but also reduces leakage. A dimpled texture can also be adapted to provide hydrodynamic pumping.

It is possible to texturize the surface of the elastomeric seal itself. However, texturing features provided on an elastomeric surface are prone to wear, as any elastomeric protrusions may be swiftly worn flat. When the texturing features are provided in the wear-resistant coating material used in the present invention, a long lasting surface texture is obtained. In one embodiment of the method of the invention, the step of processing the coating material comprises laser etching a surface texture into the coating material. Laser etching is well known in the art.

In a further embodiment, the step of processing the coating material comprises stamping a texture onto the coating material. The use of a stamp, particularly on an uncured or partially cured coating material allows for the ready forming of a surface texture without undue process complexity. Stamping the texture provides a reliable and reproducible surface texture.

In a still further embodiment, the step of processing the coating material comprises initiating a blowing agent. In a preferred example, the blowing material may be distributed throughout the coating material and only selectively activated or initiated. In this way, the bubbles only form surface texture in a desired distribution on the coated surface.

The surface texture may comprise a plurality of dimples, bumps, ridges and/or grooves. In some examples, the plurality of dimples, bumps, ridges and/or grooves form a predetermined array. In this way the pumping characteristics of the seal can be predicted and tuned to the desired purpose of the final seal.

"Dimples" as discussed herein refer to small dents or impressions in the surface of the seal. Relative to the size of the seal, the dimples are small, shallow indentations in the surface. "Bumps" are small protrusions and include pillars and projections. Ridges or grooves are extensions of dimples or bumps. All of these features may have any shape or profile on the seal surface and extending into the seal. In one embodiment, the shaping and arrangement of the dimples may resemble the surface texturing of a golf-ball, the seal provided with a distribution of concavities thereon . Furthermore, these features are distinct from mere surface roughness. They are deeper and may be specifically arranged on the surface to provide the beneficial effects. The features preferably all have substantially the same dimensions. According to another aspect of the present invention there is provided an elastomeric lip seal obtainable by the method of the present invention.

According to another aspect of the present invention there is provided a bearing comprising the elastomeric lip seal as described herein or produced by the method of the present invention.

According to another aspect of the present invention there is provided the use of a diallylamide or diallylamine coating to reduce the surface friction and/or increase the wear-resistance of an elastomeric lip seal. In particular, the compounds and seals as discussed and disclosed herein.

Examples

The effect of the present invention is demonstrated by the following non-limiting examples.

Example structures of the compounds contemplated in accordance with the coatings of the present invention are set out in table 1 .

Table 1

Two sets of five NBR rubber sheets were prepared. One sheet in each set was left uncoated, as a control sample. One sheet in each set was provided with a DLC coating, as a comparative sample. The remaining three sheets were provided with different diallylannide coatings, each coating comprising a friction modifier. The first and second set of sheets were tested with a ½ inch steel ball, loaded at a force of 2N and sliding back and forth with an amplitude of 4 mm. The sliding contact was unlubricated and the tests were performed at a temperature of 20 degrees Celsius. The first set of sheets were tested at low speed, at a frequency of 0.5 Hz. The second set of sheets were tested at high speed, at a frequency of 7.3 Hz. During the tests, the coefficient of friction (μ) was measured. Thereafter, the wear pattern was observed and graded from bad to excellent. The results are given in the table below.

Under low speed (0.5 Hz), the coefficients of the dry sliding friction (0.12 - 0.6) for the inventive coatings are lower than the value of the rubber itself, but much higher than that of the best comparable coated rubber (0.08). Under high speed (7.3 Hz), Inventive 1 has the lowest friction (0.15 - 0.26), which is similar to that of the best comparable coated rubber (0.2). Under high speed (7.3 Hz), inventive 1 has the longest wear life (> 100,000 cycles), which is better than that of the best comparable coated rubber (>25,000 cycles). Indeed, the wear track for inventive 1 was almost invisible, indicating very little wear on the coating. Inventive 1 is a coating comprising polymerised diallyamide with added PTFE powder.

Accordingly, the present invention provides an alternative coating composition to known compositions that exhibits as good or better wear properties, a low coefficient of friction and easy organic-solvent free application. When introducing elements of the present disclosure or the preferred

embodiments(s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.