Patent Application
20100048436
This application is based on and claims the benefit of priority from German patent application No. 10 2008 034 959.3, filed Jul. 25, 2008.
For tribosystems, in particular those such as are used in many technical applications, it is important to use lubricants to reduce the friction and the wear on the contact surfaces of moving parts. In this context, lubricants of different consistency can be used depending on the area of application. Lubricating oils have a liquid and flowable consistency whereas lubricating greases have a semi-solid to solid, frequently gel-like, consistency.
A characteristic feature of a lubricating grease it that a liquid oil component is taken up and retained by a thickener component. The pasty nature of a lubricating grease and its property of being spreadable and plastically easily deformable together with the property of being adhesive ensures that the lubricating grease wets the lubricating point and the lubricating effect develops at the tribologically stressed surfaces.
Lubricating greases generally consist of a thickener which is homogeneously distributed in a base oil. Additional adjuvants such as emulsifiers are frequently used so that the thicknener is stably dispersed in the base oil. Various substances are known as base oils. Organic and inorganic compounds are used as thickeners.
The most important rheological properties of a lubricating grease include the consistency or its flow limit, the avoidance of post-curing and excessive oil deposition under thermal and mechanical loading as well as a stable viscosity-temperature behaviour. Frequently at lubricating points sealed against egress of lubricating grease, a thixotropic (shear thinning) and shear-unstable behaviour of the lubricating grease is also advantageous as long as a consistent lubricating grease is only required for the mounting of corresponding components. A high degree of practical experience is required to create a lubricating grease of high usage value depending on the lubricating and equipment requirements.
Lubricating greases are frequently used in encapsulated or sealed environments in order to protect the lubricating point from water, minimise losses of lubricating grease and avoid the ingress of particles such as sand or dust. A typical application is greased joints of constant velocity shafts encapsulated with plastic bellows. In this case, the encapsulating material frequently entrains the movements of the parts which are moving with respect to one another or at least takes up vibrations. For this purpose a mobility and in most cases, also elasticity of the material is required which must not be negatively influenced by the contact or by the interaction with the lubricating grease. It has been observed, however, that conventional lubricating greases attack these encapsulating materials, with the result that these become brittle, for example, and/or are damaged by hydrolytic degradation.
Product solutions and property rights existing for the grease lubrication of constant velocity shafts are predominantly concerned with tribological questions and to a lesser degree with the provision of tribocontacts by thixotropic and/or shear-unstable greases as well as the compatibility of lubricating grease and bellows material. Among other things, the continued miniaturisation of constant velocity shafts in vehicles has led to a demand for an improved lubricant in order to satisfy the higher requirements regarding friction and tribology. The new types of lubricant according to the invention have therefore been developed because they act very well at higher ambient temperatures which are associated with the continuing vehicle development.
For the use of lubricating greases in constant velocity shafts, these must not have too-high flow limits which prevent the lubricating grease from automatically flowing back into the lubricating gap after it has been previously hurled out of the lubricating gap, for example, due to centrifugal forces.
Numerous formulation patents exist which have as their subject matter the combination of lithium, calcium, lithium complex and polyurea thickeners as consistency-giving components.
It is considered to be a shortcoming of this prior art that most lithium or lithium grease complexes and also most lithium/calcium greases have a too-high shear stability with the consequence of increased flexing work and elevated steady-state temperatures in the joint caused by this.
As a result of the greater thermal stressing of the grease and the bellows materials, this brings about a shortened lifetime of the entire system. An inadequate flow of grease causes a deficient supply and therefore inferior wetting of the tribologically stressed surfaces. As a consequence, this leads to increased wear associated with a reduction of the lifetime and a reduced efficiency caused by increased friction.
Commonly used soap thickeners, in particular the lithium and lithium complex greases widely used in joint shafts, as well as polyurea greases have a too-low alkaline buffer effect in order to prevent the formation of free acids from the grease matrix, primarily from the additives, and therefore the acidic hydrolysis of ester-group-containing polymer chains during the ageing of the grease/thermoplastic elastomer (TPE). Furthermore, the excess lithium hydroxide present in the lithium greases in turn ensures alkaline hydrolysis of the ester groups which likewise leads to a loss of the mechanical stability of the bellows. Commonly used urea greases have no alkali reserve or only a very low alkali reserve. Acidic ageing products formed in these greases can only be inadequately neutralised.
The object of the present invention is, inter alia, to minimise the disadvantages described above in regard to the bellows compatibility and the loss of efficiency and lifetime in joint shafts. The object is achieved by the subject matter of the independent claims. Preferred embodiments are the subject matter of the dependent claims or described hereinafter.
It has been found that unlike conventional lithium or calcium greases, the calcium/lithium complex soaps according to the invention (Ca/Li complex soaps) having a high fraction of calcium exhibit unexpectedly good properties when used as lubricating grease for constant velocity joints. Such products differ significantly from conventional Li/Ca greases.
The Ca/Li soaps or Ca/Li complex soaps can preferably be produced in situ using the corresponding carboxylic acid compounds and not by mixing separately produced Ca and Li soaps or Ca and Li complex soaps. According to the embodiment according to the invention, the calcium hydroxide present in excess in combination with the Ca/Li complex grease buffers the acidic ageing products without any significant alkaline hydrolysis of the ester groups of the thermoplastic elastomer occurring as a result of the comparatively low basicity and dissociation of calcium hydroxide.
It has been found that the Ca/Li complex greases are preferably produced so that the amount of lithium compound and calcium compound required for the conversion are determined so as to ensure that the more reactive lithium compound (preferably LiOH in the present case) reacts as completely as possible with the carboxyl groups of the carboxylic acids used and the remaining carboxyl groups not reacted with the lithium compound are reacted, as it were, in excess with calcium compounds, preferably Ca(OH)2 in the present case.
In addition, the calcium soaps present in excess should lead to a reduced mechanical stability or lower flow limit after running in the joint shafts and thus lead to an improved supply of friction points in the constant velocity joints.
The product formulation contains an additional complexing agent in order to compensate for the drop in dropping point observed due to the increased calcium fraction, e.g. to below 180° C. The composition according to the invention is therefore designated as Ca/Li complex grease or Ca/Li complex soap grease and not as Ca/Li grease or Ca/Li soap grease.
The use of excess calcium hydroxide and calcium soaps counteracts the acidic ageing products such as are formed due to the use of additives. The ageing products contribute to the destruction of the bellows.
The composition according to the invention is composed of:
The wt. % data relate to the overall composition and apply in each case independently of one another. A component assigned to one of the groups a), b), c) or d) cannot simultaneously be a component of another group a) to d).
The composition is further characterized by a calcium/lithium molar ratio of 1.0 to 5.0 to 1, preferably 1.2 to 3.0 to 1 and in particular 1.4 to 2.4 to 1, wherein the ratio is determined by all the calcium and lithium compounds present, i.e. the Ca(OH)2 present is also included, for example.
In particular, the thickener is used such that the composition contains so much thickener that a cone penetration value (worked penetration) of 265 to 385 mm/10 (at 25° C.), preferably 285 to 355 mm/10, is obtained (determined in accordance with DIN ISO 2137 or ASTM D 0217-97).
The usual lubricating oils which are liquid at room temperature are used as base oil. The base oil preferably has a kinematic viscosity of 20 to 2500 mm2/s, in particular of 40 to 500 mm2/s at 40° C.
The base oils can be classified as mineral oils or synthetic oils. Considered as mineral oils, for example, are naphthene-based mineral oils and paraffin-based mineral oils, as classified according to API Group I. Chemically modified low-aromatic and low-sulphur mineral oils having a low fraction of saturated compounds and improved viscosity/temperature behaviour compared with group I oils, classified according to API Group II and III, are also suitable.
Named as synthetic oils are polyethers, esters, polyalphaolefins, polyglycols and alkyl aromatic compounds and mixtures thereof. The polyether compound can comprise free hydroxyl groups but can also be completely etherised or terminal group esterified and/or produced from a starting compound having one or more hydroxy and/or carboxyl groups (—COOH). Polyphenyl ethers, optionally alkylated, are also possible as sole components or better as mixed components. It is suitable to use esters of an aromatic di-, tri- or tetracarboxylic acid, with one or a mixture of C2 to C22 alcohols, esters of adipic acid, sebacic acid, trimethylolopropane, neopentylgycol, pentaerythrite or dipentaerythrite with aliphatic branched or unbranched, saturated or unsaturated C2 to C22 carboxylic acids, C18 dimeric acid esters with C2 to C22 alcohols, complex esters, as single components or in any mixture.
The Ca/Li soap is a mixture of calcium salts and lithium salts of one or more saturated or unsaturated monocarboxylic acids having 10 to 32 carbon atoms, optionally substituted, in particular having 12 to 22 carbon atoms, particularly preferably corresponding hydroxycarboxylic acids. Suitable carboxylic acids are, for example, lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid or behenic acid as well as preferably 12-hydroxystearic acid. Instead of the free acidic groups, corresponding lower alcohol esters can also be used while saponifying, e.g. corresponding triglycerides as well as methyl, ethyl, propyl, isopropyl or sec.-butyl esters of the acid/hydroxy acid in order to achieve a better dispersion.
Due to the presence of a complexing agent, the Ca/Li soap becomes a Ca/Li complex soap. Complexing agents in the sense of the present invention are:
The complexing agent (a) is preferred. Particularly suitable as monocarboxylic acids are acetic acid and propionic acid. Likewise suitable are hydroxybenzoic acids such as parahydroxybenzoic acid, salicylic acid, 2-hydroxy-4 hexylbenzoic acid, metahydroxybenzoic acid, 2,5-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid (gentisinic acid), 2,6-dihydroxybenzoic acid (gamma-resorcylic acid) or 4-hydroxy-4-methoxybenzoic acid. Particularly suitable as dicarboxylic acids are adipic acid (C6H10O4), sebacic acid (C10H18O4), azelaic acid (C9H16O4) and/or 3-tert.-butyl-adipic acid (C10H18O4).
Metaborate, diborate, tetraborate or orthoborate, such as for example, monolithium orthoborate or calcium orthoborate can be used, for example, as borate (b). Possible phosphates are alkali (preferably lithium), as well as alkaline earth (preferably calcium), dihydrogen phosphate, hydrogen phosphate or pyrophoshate.
In addition, bentonites such as montmorillonite (whose sodium ions are optionally exchanged or partially exchanged by ammonium ions), aluminosilicates, alumina, silicic acid (e.g. Aerosil), copper phthalocyanines or also di- and polyureas can optionally be used as co-thickeners.
The compositions according to the invention further contain additives as adjuvants. Usual additives in the sense of the invention are antioxidants, antiwear agents, anticorrosives, detergents, dyes, lubricity improvers, viscosity additives, friction modifiers and high-pressure additives.
As examples, mention may be made of:
In addition, the lubricating grease compositions according to the invention contain usual additives against corrosion, oxidation, and for protection against metal influences which act as chelate compounds, radical collector, UV converters, reaction layer formers and the like.
For example, polymer powders such as polyamides, polyimides or PTFE, graphite, metal oxides, boron nitride, metal sulfides such as, for example, molybdenum disulfide, tungsten disulfide or mixed sulfides based on tungsten, molybdenum, bismuth, tin and zinc, inorganic salts of alkali and alkaline earth metals such as, for example, calcium carbonate, sodium and calcium phosphates, can be used as solid lubricants. Solid lubricants can be divided into the following four groups: compounds having a layer lattice structure such as molybdenum disulfide and tungsten disulfide, graphite, hexagonal boron nitride and some metal halides; oxide or hydroxide compounds of the transition and rare earth metals or their carbonates or phosphates; soft metal and/or plastics.
According to a further embodiment of the invention, the sealing material, including encapsulating materials which are in contact with the lubricant, is a polyester, preferably a thermoplastic copolyester elastomer comprising hard segments with crystalline properties and a melting point above 100° C. and soft segments having a glass transition temperature of less than 20° C., preferably less than 0° C.
Particularly suitable are polychloroprene rubber and thermoplastic polyester (TPE), thermoplastic polyether ester (TEEE=thermoplastic ether ester elastomer). The latter are available on the market under the trade names Arnitel® from DSM, Hytrele from DuPont and PIBI-Flex® from P-Group.
The hard segments are derived, for example, from at least one aliphatic diol or polyol and at least one aromatic di- or polycarboxylic acid, the soft segments having elastic properties, for example, from ether polymers such as polyalkylene oxide glycols or non-aromatic dicarboxylic acids and aliphatic diols. These compounds are designated, for example as copolyether esters.
Copolyether ester compositions are used in components, for example when the component produced therefrom is exposed to frequent deformation or vibrations. Very well known applications in this respect are bellows or spring bellows for protecting drive shafts and transmission shafts, steering columns and suspension units as well as sealing rings. In such applications, the material comes frequently or continuously in contact with lubricants such as lubricating greases.
Technically, it is possible to proceed by producing the bellows by injection blow moulding, injection extrusion or extrusion blow moulding, wherein annular parts made of rubber are optionally inserted beforehand in the mould at the two future clamping points.
The resistance of the copolyether ester composition to the effects of oils and greases is one of the reasons for its widespread use along with its easy processability in relatively complex geometries.
A lubricating grease according to the present invention should preferably, but not exclusively, be used for lubricating constant velocity joints (homokinetic joints or “constant velocity joint” CVJ) such as are used, for example, in drive shafts for motor vehicles. The task of a constant velocity joint is the uniform transmission of torque from one shaft to a second shaft mounted at an angle thereto at articulation angles of up to 50°.
In vehicle construction constant velocity joints are used particular in front-wheel drive and all-wheel drive vehicles and are generally divided into two groups: constant velocity fixed joints and constant velocity slip joints. The group of constant velocity fixed joints includes ball joints (“ball joints” BJ) and undercut free constant velocity joints (“undercut free” UJ). The group of constant velocity slip joints is formed by tripod constant velocity joints (“tripod joints” TJ), double offset joints (“double offset joints” DOJ) and cross groove constant velocity joints (“cross groove joints” LJ). In contrast to constant velocity fixed joints, constant velocity slip joints not only ensure uniform torque transmission at different articulation angles but additionally provide length compensation of the drive shaft connecting the transmission to the wheel.
Since constant velocity joints are almost exclusively lubricated with lubricating greases, they are usually encapsulated with a bellows to prevent egress of lubricating grease or ingress of dirt or moisture. This bellows is usually designed in one piece and inverted over the drive shaft after greasing or mounting the constant velocity joint.
The property profile of the different greases as produced hereinafter is characterized in detail by the figures and Table 1.
In detail the figures show:
The manufacture of the lubricating greases can take place, for example, as follows: Mixing the salt/metal compound into the carboxylic acid compound which can optionally be diluted with the base oil component and optional simultaneous heating of the mixture to a temperature above 70° C. to form a thickened lubricating grease product, cooling the lubricating grease product and optionally adding water, allowing shear forces to act on the mixture, for example, using a crosshead mill, a high-pressure homogeniser and/or a triple roller.
For this purpose, in a first step a vessel with an agitator at a rotational speed of 0 to 150 rpm was charged with 3 to 40 wt. % of the educt components of the thickener and 20 to 90 wt. % of the base oil, relative to the total weight of the finished lubricating grease composition.
According to a preferred embodiment of the invention, the thickener is synthesized in situ in the base oil, preferably under pressure and elevated temperature in a closed reaction vessel such as an autoclave.
Description of Production Example A to C (Comparative Example)
The production of the exemplary formulations A-C is based on known and usual methods for producing corresponding thickeners.
Description of Production Example E
In a reactor 1,540 g of 12-hydroxystearic acid (12-HSA) and 560 g of sebacic acid were placed in 12,000 g of a base oil mixture and heated until the 12-HSA melts whilst agitating. In the next step, 125 g of LiOH and 295 g of Ca(OH)2 were added. The Ca(OH)2 was dissolved in 300 g of water before adding to the reactor. The formulation was heated in a fixed temperature program to the intended maximum process temperature whilst agitating. In the cooling phase additives were added to the formulation at specified temperatures. After adjusting the formulation to the desired consistency by adding 5,400 g of base oil mixture, the end product was homogenized by means of a crosshead mill.
Description of production Example D and G: the exemplary formulations were produced by analogy with exemplary formulation E by varying the amounts of LiOH and Ca(OH)2.
It should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2008 034 959.3 | Jul 2008 | DE | national |
