Cold-proof Grease Composition For Reducing Noise

Abstract

The invention provides a cold-proof grease composition for reducing noise. In certain embodiments, the invention provides a grease composition having an improved noise-reduction effect without sacrificing lubricant adhesiveness at high and low temperatures. In one embodiment, the grease composition comprises a synthetic hydrocarbon oil as a base oil, a lithium-based thickener, and a poly(methyl acrylate) (PMA)-based copolymer.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. .sctn.119 to Korean Patent Application No. 10-2012-0120174, filed on Oct. 29, 2012, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

[0002] (a) Technical Field

[0003] The invention provides a cold-proof grease composition. In certain embodiments, the grease composition of the invention is useful for reducing noise. In particular, the grease composition of the invention comprises a synthetic hydrocarbon oil as a base oil, a lithium-based thickener, and a poly(methyl acrylate) (PMA)-based copolymer. The grease composition of the invention offers an improved noise-reduction effect without sacrificing lubricant adhesiveness at high and low temperatures.

[0004] (b) Background Art

[0005] Grease is generally used as a lubricant for automotive components. Although a grease material having high viscosity helps to reduce noise, operation of the components is impaired due to an increased load. Consequently, an operation failure may occur. In general, the viscosity of grease decreases at high temperatures and increases at low temperatures. Generally, a grease material with a high viscosity at high temperatures also has a high viscosity at low temperatures. As for the grease used in an automobile which is exposed to various temperature environments ranging from low to high temperatures, if a grease material having a high viscosity is used to reduce noise, an operation load may be induced at low temperatures. And, if a grease material having a low viscosity is used to ensure operability at low temperatures, operation noise may increase. As the result, the basic performance and durability of the related components may be compromised at high temperatures.

[0006] Accordingly, development of a grease material with a viscosity having a decreased dependency on temperatures is necessary for achieving an improvement of the performance and durability.

[0007] To minimize the change in viscosity as the temperature varies, costly ester base oils, silicone base oils or fluoroether base oils are often used as the base oil of grease. However, due to limitations placed by electric parts and a high cost associated with the above base oils, a poly-.alpha.-olefin (PAO) synthetic base oil, which exhibits good cost effectiveness, is commonly used. Further, a method of increasing viscosity at high temperatures using polymer materials (such as, polyisobutylene, polybutylene (PB), etc.) has also been developed. U.S. Pat. No. 5,116,522 and U.S. Pat. No. 5,108,635 disclose a grease composition including PMA or a lithium-based thickener in a base oil. However, the grease composition has limited effects in maintaining durability of the components used in the latest high-power, high-efficiency automobiles, and in reducing noise.

[0008] Korean Patent No. 513,625 discloses the use of PAO as a base oil, PMA as a polymer additive, and a lithium-based soap as a thickener for a grease material. Nevertheless, the resulting grease material is highly temperature-dependent compared with that of the invention and offers limited effects in improving durability of automotive parts under various temperature environments, maintaining operability of the automotive parts, and reducing noise. The details are described in examples infra. Korean Patent No. 135,414 discloses a grease composition for a ball joint. The grease composition includes a synthetic base oil comprising a mixture of poly-.alpha.-olefin with a high viscosity index and poly-.alpha.-olefin with a low viscosity index, a lithium-based soap as a thickener, a PMA-based copolymer, and other supplementary additives. The durability at high temperatures and noise reduction effects are unsatisfactory despite that a relatively high weight percentage (25-50%) of the PMA-based copolymer is used. Satisfactory result has not been obtained even after an antioxidant, an extreme pressure additive, etc. are added to improve the durability at high temperatures.

SUMMARY

[0009] The invention is based on the finding that use of a synthetic hydrocarbon oil, instead of an ester oil, as a base oil and a lithium-based thickener results in an improved service life of grease at high temperatures while degradation is minimized at high temperatures. In particular, the invention is based on the finding that use of a PMA-based copolymer results in an improved operability at low temperatures and an improved durability at high temperatures, thereby providing effects of reducing noise independent of a change in temperature.

[0010] In one embodiment of the invention, a PMA polymer material, instead of a PIB/PB polymer material, is added to a PAO-based synthetic base oil. When PAO with a viscosity of 4-6 cSt at 100.degree. C. is used and when an asterisk type of PMA, instead of a linear type PMA, is used as the PMA-based copolymer, the dependency of the viscosity on temperature can be minimized.

[0011] Accordingly, the invention provides a less temperature-dependent cold-proof grease composition for reducing noise. In certain embodiments, the grease composition of the invention includes a synthetic hydrocarbon oil as a base oil, a PMA-based copolymer, and a lithium-based thickener.

[0012] In an aspect, the present invention provides a cold-proof grease composition for reducing noise comprising:

[0013] 50-75 wt % of a base oil including a synthetic hydrocarbon oil;

[0014] 10-30 wt % of a poly(methyl acrylate) (PMA)-based copolymer; and

[0015] 10-20 wt % of a lithium-based thickener.

DETAILED DESCRIPTION

[0016] Reference will now be made in detail hereinafter to various embodiments and examples of the invention. Further, the invention is illustrated in the accompanying drawings and described below. While the invention is described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

[0017] The invention provides a grease composition comprising a synthetic hydrocarbon oil, instead of an ester oil, as a base oil, a PMA-based copolymer, and a lithium-based thickener. The grease composition of the invention improves compatibility and adhesivity among various materials including plastics, improves the service life of components to which the composition is applied under high temperatures and low temperatures, and ensures a low-noise operation and an improved durability.

[0018] The base oil used in the invention can be any base oil commonly used in grease. In certain embodiments, base oils having no effects on metals, plastics or rubbers may be used. Such base oils including, for example, a synthetic hydrocarbon oil, a mineral oil and a mixture thereof. In one embodiment, a base oil consisting essentially of a synthetic hydrocarbon oil is used. The synthetic hydrocarbon oil may comprise poly-.alpha.-olefin (PAO) or a copolymer of .alpha.-olefin and olefin.

[0019] In one embodiment, the synthetic base oil of the invention has a dynamic viscosity of 4-6 cSt at 100.degree. C. and a pour point of -50.degree. C. or lower at low temperatures. If the dynamic viscosity is lower than 4 cSt, the composition may evaporate easily, rendering the heat resistance insufficient. On the other hand, if the dynamic viscosity exceeds 6 cSt, torque and heat generation may increase.

[0020] According to the invention, the base oil may be used in an amount of 50-75 wt %, or 58-72 wt %. If the content of the base oil is less than 50 wt %, the grease composition may be solidified easily. And, if it exceeds 75 wt %, the composition may be liquefied at high temperatures.

[0021] In certain embodiments of the invention, a PMA-based copolymer is used to improve the viscosity index and adhesivity of the base oil. In certain embodiments, the PMA-based copolymer is added in an amount of 10-30 wt %, or 13-24 wt %, of the entire grease composition. If the content of the PMA-based copolymer is less than 10 wt %, there would be an insufficient improvement of the viscosity index and adhesivity resulted from the addition of the PMA-based copolymer. And, if the content of the PMA-based copolymer exceeds 30 wt %, operability and noise problems may occur at low temperatures due to an increased viscosity.

[0022] In one embodiment, the PMA-based copolymer used in the invention is a copolymer containing a vinyl (--C.dbd.C--) group or a carbonyl (--COO--) group. The copolymer is represented by the following chemical formulas 1-4:

R.sub.1COOR.sub.2 (1)

R.sub.3COOR.sub.4COOR.sub.5 (2)

R.sub.6CH.dbd.CHR.sub.7 (3)

R.sub.8CH.dbd.CHR.sub.9COOR.sub.10 (4)

[0023] In the chemical formulas 1-4, R.sub.1-R.sub.10 is a hydrocarbon group, including, such as, a saturated aliphatic group, an unsaturated aliphatic group, an aromatic group or a substituent group. In a separate embodiment, the PMA-based copolymer of the invention is an asterisk type, not a linear type. The asterisk type copolymer has a radial or star-shaped structure and, because of its structural characteristics, exhibits an increased viscosity, an increased viscosity index and superior low-temperature properties when dissolved in oil. The improvement of the viscosity index and adhesivity of the base oil resulted from the addition of the PMA-based copolymer means that the viscosity index of the base oil comprising 20 wt % of the PMA-based copolymer is 200 or greater at 40.degree. C. and 100.degree. C. The viscosity index is calculated by measuring the viscosity of the base oil using a viscometer. The dynamic viscosity is measured according to KS M 2014.

[0024] In other embodiments of the invention, a lithium-based thickener is used as the thickener of grease. The lithium-based thickener is used in an amount of 10-20 wt %, or 12-17 wt %, of the entire grease composition. If the content of the lithium-based thickener is less than 10 wt %, viscosity decreases at both high and low temperatures. And, if the content of the lithium-based thickener exceeds 20 wt %, viscosity becomes excessively high at both high and low temperatures, resulting in solidification of the composition.

[0025] The lithium-based thickener that may be used in the invention are represented by the following chemical formula 5:

(R.sub.11--COOLi).sub.n (5)

[0026] In the chemical formula 5, R.sub.11 is a C.sub.6-C.sub.20 aliphatic hydrocarbon group and n is an integer. If the number of carbons in R.sub.11 is less than 6, the grease composition may not be thickened sufficiently. And, if the number of carbons in R.sub.11 exceeds 20, the heat resistance of the grease composition may be poor.

[0027] The lithium-based thickener can be obtained by reacting lithium hydroxide with a fatty acid compound. To ensure that no reactive fatty acid groups remain in the final product, lithium hydroxide and the fatty acid compound containing fatty acid groups in a mole almost equivalent to the lithium hydroxide are mixed. The lithium hydroxide and the fatty acid compound may be reacted in the base oil. Alternatively, a previously synthesized fatty acid lithium-based soap thickener may be mixed with the base oil. In a specific embodiment, the former method is used to achieve a better grease stability.

[0028] The grease composition according to the invention may further comprise additives commonly added to grease. Such additives including, such as, an extreme pressure additive, an antioxidant, a corrosion inhibitor, etc.

[0029] Further, if so desired, supplementary additives, such as, a rust inhibitor, a metal deactivator, and a viscosity index improver, may also be added.

[0030] An extreme pressure additive that is commonly used may be added to improve the load resistance or extreme pressure properties. For example, the following compounds may be used as extreme pressure additives. In one embodiment, the extreme pressure additives are organometals including an organomolybdenum compound, such as, molybdenum dithiocarbamate, molybdenum dithiophosphate, etc.; an organozinc compound such as, zinc dithiocarbamate, zinc dithiophosphate, zinc phenate, etc.; an organoantimony compound, such as, antimony dithiocarbamate, antimony dithiophosphate, etc.; an organoselenium compound, such as, selenium dithiocarbamate, etc.; an organobismuth compound, such as, bismuth naphthenate, bismuth dithiocarbamate, etc.; an organoiron compound, such as, iron dithiocarbamate, iron octylate, etc.; an organocopper compound, such as, copper dithiocarbamate, copper naphthenate, etc.; an organotin compound, such as, tin maleate, dibutyltin sulfide, etc.; an organic sulfonate, phenate or phosphate of an alkali metal or an alkaline earth metal; an organometal compound of gold, silver, titanium, cadmium, etc. In another embodiment, the extreme pressure additives are sulfur compounds including a sulfide or polysulfide compound, such as, dibenzyl disulfide, etc.; a sulfurized oil; an ashless carbamate compound; a thiourea compound; and a thiocarbonate compound, or the like.

[0031] Further, the extreme pressure additive can also be phosphate-based, which includes a phosphate compound, such as, trioctyl phosphate, tricresyl phosphate, etc.; or a phosphate ester compound, such as, phosphoric acid ester, phosphorus ester, phosphorus acid ester, etc. Also, a halogen-based extreme pressure additive can be used in the invention. The halogen-based extreme pressure additives include, such as, chlorinated paraffin, etc. In addition, a solid lubricant, such as, molybdenum disulfide, tungsten disulfide, graphite, PTFE, antimony sulfide, boron nitride, boron, etc., may be used. In a specific embodiment, the invention uses a dithiocarbamate compound or a dithiophosphate compound as the extreme pressure additive.

[0032] The antioxidant of the invention may be selected from the group of an aging resister, an antiozonant, and an antioxidant that can be added to rubber, plastics, lubricating oil, etc. For example, the antioxidant of the invention can be an amine compound, such as, phenyl-1-naphthylamine, phenyl-2-naphthylamine, diphenyl-p-phenylenediamine, dipyridylamine, phenothiazine, N-methylphenothiazine, N-ethylphenothiazine, 3,7-dioctylphenothiazine, p,p'-dioctyldiphenylamine, N,N'-diisopropyl-p-phenylenediamine, N,N'-di-sec-butyl-p-phenylenediamine, etc., or a phenol compound, such as, 2,6-di-tert-dibutylphenol, etc., or the like.

[0033] The corrosion inhibitor of the invention can be, for example, an ammonium salt of a sulfonic acid, an organic sulfonate, carbonate, phenate or phosphate of an alkali or alkaline earth metal (such as, barium, zinc, calcium, magnesium, etc.), an alkyl or alkenyl succinic acid derivative (such as, alkyl or alkenyl succinic acid ester), a partial ester of a polyol (such as, sorbitan monooleate), a hydroxy fatty acid (such as, oleoylsarcosine), a mercapto fatty acid (such as, 1-mercaptostearic acid) or a metal salt thereof, a higher fatty acid (such as, stearic acid), an ester of a higher alcohol (such as, isostearyl alcohol) with a higher fatty acid, a thiazole compound (such as, 2,5-dimercapto-1,3,4-thiadiazole, 2-mercaptothiadiazole, etc.), an imidazole compound (such as, 2-(decyldithio)-benzimidazole, benzimidazole, etc.), a disulfide compound (such as, 2,5-bis(dodecyldithio)benzimidazole), a phosphate ester compound (such as, trisnonylphenyl phosphite), a thiocarbonate ester compound (such as, dilauryl thiopropionate), or the like. Also, a nitrite compound may be used.

EXAMPLES

[0034] The invention is described in more detail through examples. The following examples are for illustrative purposes only. It will be apparent to those skilled in the art not that the scope of this invention is not limited by the examples.

Examples 1-5 and Comparative Examples 1-7

[0035] A cold-proof grease composition for reducing noise was prepared by mixing a base oil, a lithium-based thickener and a PMA-based copolymer as described in Table 1. Poly-.alpha.-olefin oil (ExxonMobil, viscosity: 4-6 cSt at 100.degree. C.), which is a synthetic hydrocarbon oil, was used as the base oil. The lithium-based thickener was obtained by reacting lithium hydroxide with 12-hydroxystearic acid. And, asterisk type PMA (Lubrizol, viscosity: 500-800 cSt at 100.degree. C.) was used as the PMA-based copolymer.

TABLE-US-00001 TABLE 1 Examples Comparative Examples (wt %) 1 2 3 4 5 1 2 3 4 5 6 7 Synthetic hydrocarbon oil 63.3 71 59 62.2 64.9 73 55.5 66.3 60.3 54.3 56.3 62.1 Methacrylate polymer A 22.2 14.5 26.5 21.8 22.8 9.2 32 22.2 22.2 -- -- -- Methacrylate polymer B -- -- -- -- -- -- -- -- -- 24.6 -- Methacrylate polymer C -- -- -- -- -- -- -- -- -- 25.5 -- Methacrylate polymer D -- -- -- -- -- -- -- -- -- -- -- 23.4 Li-soap 14.5 14.5 14.5 16 12.5 14.5 14.5 9 21.5 14.5 14.5 14.5

Test Example

[0036] Penetration of the base oil was measured according to ASTM D217.

[0037] Apparent viscosity of grease was tested according to DIN 51810.

[0038] Low-temperature torque of grease was tested according to KS M 2130.

[0039] Operation noise was tested by applying the grease composition of the invention or samples prepared according to the existing art to a door latch. Noise is compared.

TABLE-US-00002 TABLE 2 Examples Comparative Examples (wt %) 1 2 3 4 5 1 2 3 4 5 6 7 Penetration 290 280 285 267 310 295 290 325 245 294 289 295 Apparent viscosity (25.degree. C.) 2.6 2.3 2.8 2.8 2.4 1.9 3.2 1.7 2.9 2.3 2.5 2.1 Apparent viscosity (-40.degree. C.) 14.9 13.2 17.2 18.6 13.0 12.4 22.3 11.7 22.0 35.0 40.0 37.0 Low- startup 2379 2156 2689 2415 2015 1956 3318 1828 3508 5527 6082 5723 temperature revolution 856 795 902 895 752 698 1084 611 1052 1858 2130 2017 torque (-40.degree. C.)

[0040] Table 2 shows the result of penetration measurement, change in viscosity at room temperature and low temperature, and low-temperature torque of the grease compositions of Examples 1-5 and Comparative Examples 1-7. It can be seen that the compositions of Examples 1-5 do not show significant viscosity increase at low temperatures, compared to the prior art samples at low temperatures.

TABLE-US-00003 TABLE 3 Existing The present art invention Improvement Viscosity (Pa s) room 2.02 2.6 0.58 (29%) temperature low 5.96 14.9 8.94 (150%) temperature Operation Latch closed 72.1 67.1 5.0 (6.9%) noise locking open 78.0 75.1 2.9 (3.7%) Ac- closed 71.0 65.2 5.8 (8.1%) tuator open 64.5 60.6 3.9 (6.0%)

[0041] The grease composition according to the existing art contains poly-.alpha.-olefin oil as the synthetic hydrocarbon oil and a lithium-based thickener obtained by reacting lithium hydroxide with 12-hydroxystearic acid. A low-viscosity poly-.alpha.-olefin oil was used to ensure good low-temperature properties. Although the grease composition according to the existing art showed good low-temperature properties, operation noise occurred because of the low viscosity. In contrast, the grease composition according to the invention showed an improvement in reducing operation noise, as its viscosity was higher than that of the prior art composition both at the room temperature and low temperatures. Table 3 compares the results of the measured viscosity and operation noise of the prior art grease compositions and the grease compositions of the invention at the room temperature and low temperatures. It was expected that the grease composition of the invention is effective in reducing noise as it has an increased viscosity compared to that of the prior art composition. This noise-reduction effect of the grease composition of the invention was confirmed in latch locking and actuator operation, when the operation noise was tested for a door latch.

[0042] The grease composition according to the invention comprises a synthetic hydrocarbon oil, instead of ester oil, as a base oil. The synthetic hydrocarbon oil is effective in preventing brittleness (cracking) and enhancing durability without incurring adverse effects on plastics and rubbers. Further, the addition of a lithium-based thickener improves mechanical stability and durability. The addition of a PMA-based copolymer, which improves viscosity index and adhesivity of the base oil, helps to prevent degradation of grease, and greatly improves the service life of grease at high temperatures and the lubricating performance at very low temperatures. Furthermore, the grease composition ensures low noise and vibration under all temperature and humidity conditions to which high-power, high-efficiency automobiles may be exposed.

[0043] The invention has been described in detail with reference to specific embodiments thereof. However, it will be appreciated by those skilled in the art that various changes and modifications may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A cold-proof grease composition for reducing noise comprising: 50-75 wt % of a base oil comprising a synthetic hydrocarbon oil; 10-30 wt % of a poly(methyl acrylate) (PMA)-based copolymer; and 10-20 wt % of a lithium-based thickener.

2. The cold-proof grease composition for reducing noise according to claim 1, wherein the synthetic hydrocarbon oil is an aliphatic hydrocarbon oil with a viscosity of 4-6 cSt at 100.degree. C.

3. The cold-proof grease composition for reducing noise according to claim 2, wherein the aliphatic hydrocarbon oil comprises poly-.alpha.-olefin (PAO) or a copolymer of .alpha.-olefin and olefin.

4. The cold-proof grease composition for reducing noise according to claim 1, wherein the PMA-based copolymer is an asterisk type having a radial or star-shaped structure and has a dynamic viscosity of 500-800 cSt at 100.degree. C.

5. The cold-proof grease composition for reducing noise according to claim 4, wherein the PMA-based copolymer comprises a styrene-alkyl methacrylate copolymer or a styrene-alkyl methacrylate copolymer.

6. The cold-proof grease composition for reducing noise according to claim 1, wherein the lithium-based thickener is obtained by reacting lithium hydroxide with 12-hydroxystearic acid or by reacting lithium hydroxide with a hydrogenated castor oil.

7. The cold-proof grease composition for reducing noise according to claim 1, further comprising 3 parts by weight of the grease composition of one or more additive selected from a group consisting of an extreme pressure additive, an antioxidant, a corrosion inhibitor, a rust inhibitor, a metal deactivator and a viscosity index improver.