Patent Publication Number: US-2012028857-A1

Title: Automobile transmission oil composition with improved low-temperature transmission performance

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2010-0074076 filed Jul. 30, 2010, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (a) Technical Field 
     The present disclosure relates to an automobile transmission oil and a method of lubricating an automobile transmission using the oil. More particularly, it relates to a manual transmission oil composition containing a polyalphaolefin synthetic base oil and a viscosity modifier. 
     (b) Background Art 
     In a typical vehicle, a transmission is installed between a clutch and a drive shaft or between a clutch and a final reduction gear. The transmission converts the engine&#39;s output by adjusting the speed and torque so as to drive the wheels with rotational speeds and forces suitable for operating the vehicle. A manual transmission changes the rotational speed and force transmitted from the engine through a gear shaft which is in connection with the clutch such that, according to the running conditions of the vehicle and the manipulation of the clutch, the power is converted and transmitted to the drive wheels, thereby providing the speed and torque that the driver demands. 
     The manual transmission can be equipped with a synchronizer to facilitate the driver&#39;s gear shift. A synchronizer is a device for synchronizing the speed of a shift gear with the speed of a clutch gear during gear shifting to provide smoother shifting. In some embodiments, the synchronizer includes a synchromesh which brings the speed of the shift gear in synchronization with the speed of the clutch gear before engaging the gears, for example by the inclined contact of a synchromesh and a shift gear. 
     Automobile transmissions, including manual transmissions, importantly use a transmission oil, which has a main function of lubricating the moving parts. However, as it is known, the viscosity of oil varies with temperature change and, thus, the viscosity of the transmission oil can vary depending on factors such as the time of year and whether and/or how much the vehicle has been warmed up. If the viscosity of the transmission oil is significantly increased, such as during cold winter conditions, synchronization achieved by friction during contact is impeded, thereby reducing shifting performance. Further, in conditions where the outside temperature is very low, such as −20 to −50° C., gear shifting may even be prevented. 
     In attempt to deal with this problem, Korean Patent No. 298,035, Korean Patent No. 706,434, Korean Patent Publication No. 2008-0109877, etc. provides a method for reducing the viscosity of transmission oil at low temperatures. However, the method provided simply reduces the overall viscosity, and thus the viscosity at high temperatures is also reduced. As a result, the durability of the transmission is deteriorated and significant rattle noise occurs. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE DISCLOSURE 
     The present invention features an automobile transmission oil composition which is provided with a low viscosity at low temperatures, while reducing rattle noise and maintaining excellent transmission durability even at high temperatures. The transmission oil composition is useful for lubricating and operating various types of transmissions, including a manual transmission. According to an aspect of the present invention, an automobile transmission oil composition comprises a synthetic base oil and a viscosity index improver. Preferably, polyalphaolefin (PAO) synthetic base oil is used as the base oil and polyalkylmethacrylate is used as the viscosity index improver. The combination of predetermined amounts of a synthetic base oil and a viscosity index improver in accordance with the present invention provides an oil having a low viscosity at low temperatures as compared to other transmission oils having the same viscosity grade (SAE 75W85 grade). 
     In an exemplary embodiment, a manual transmission oil composition is provided which comprises about 78 to 80 wt % of polyalphaolefin synthetic base oil, about 10 to 12 wt % of polyalkylmethacrylate, and one or more additives. In certain aspects, the polyalkylmethacrylate has a number-average molecular weight of about 300,000 to 400,000. Preferably, the composition includes about 8 to 12 wt % of one or more additives. These additives can include any known additives commonly included in transmission oils such as, for example, anti-wear agents, metallic detergent-dispersants, friction modifiers (particularly phosphate ester friction modifiers), dispersants (particularly bis-succinimide ashless dispersants), extreme pressure additives, and mixtures thereof. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
     The above and other features of the invention are discussed infra. 
    
    
     DETAILED DESCRIPTION 
     Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that 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. 
     One aspect of the present invention provides an automobile transmission oil composition comprising a synthetic base oil, preferably polyalphaolefin (PAO) synthetic base oil, and a viscosity index improver. In certain embodiments, as a viscosity index improver, polyalkylmethacrylate is used, particularly polyalkylmethacrylate having a number-average molecular weight ranging from about 300,000 to 400,000. It is noted that synthetic base oils and viscosity index improvers are known and, thus, it is possible to use known materials rather than or in combination with the specific synthetic base oils and viscosity index improvers specifically described herein. Such transmission oil compositions are able to reduce the flow resistance at low temperatures, which beneficially improves the shifting performance at low temperatures. The present transmission oil compositions also provide improved transmission durability as well as enhanced performance features (such as shear stability, friction characteristics, thermal stability, and oxidation stability). Further, the rattle noise which often results at high operating temperatures is reduced when using the present compositions. 
     In one embodiment, a manual transmission oil composition is provided wherein the polyalphaolefin (PAO) synthetic base oil has an average kinematic viscosity ranging from about 3 to 8 cSt at 100° C. It has been found that when the kinematic viscosity at 100° C. is less than 3 cSt, the amount of oil evaporated during use increases under high temperature and dry conditions. It has also been found that when the kinematic viscosity at 100° C. exceeds 8 cSt, the viscosity of the oil significantly increases, which reduces the shifting performance and fuel efficiency at low temperatures. As such, it is preferable that the synthetic base oil have the kinematic viscosity in the above range so as to reduce or eliminate these problems. In some embodiments, the polyalphaolefin (PAO) synthetic base oil further has a viscosity index of about 130 or higher, and a pour point of about −60° C. 
     Moreover, The PAO synthetic base oil is generally contained in a large amount of the transmission oil composition (i.e., more than about 50 wt % relative to the total weight of the lubricating oil composition). Preferably, the transmission oil composition contains at least about 75 wt %, more preferably at least about 78 wt %, and preferably no greater than about 80 wt % PAO synthetic base oil. In a preferred embodiment, the transmission oil composition contains about 78 to 80 wt % PAO synthetic base oil, The transmission oil composition further includes a viscosity index improver. Without being bound by theory, it is believed that by combining the synthetic base oil with a viscosity index improver in accordance with the present invention, the viscosity of the manual transmission oil is increased and the increase in viscosity at low temperatures is inhibited or even prevented, which thereby improves the viscosity index of the oil. A particularly preferred viscosity index improver is polyalkylmethacrylate. Suitable polyalkylmethacrylate may have at least 4 polymer chains, and in some preferred embodiments may have 4 to 10 polymer chains. In an exemplary embodiment, the polyalkylmethacrylate contains about 60 to 80 wt % of C 12  to C15 alkyl methacrylate and about 20 to 40 wt % of 2-ethylhexyl methacrylate. Preferred polyalkylmethacrylates are those which have a kinematic viscosity of about 650 cSt or higher at 100° C. In preferred embodiments, the transmission oil compositions contains about 10 to 12 wt % polyalkylmethacrylate. It has been found that when the amount of polyalkylmethacrylate used is less than 10 wt %, the viscosity is not adequately increased at high and low temperatures, which causes an excessive increase in viscosity at low temperatures. It has further been found that if the amount of polyalkylmethacrylate used exceeds 12 wt %, the viscosity at high and low temperatures is increased at the same time, which causes a reduction in viscosity due to shear as time goes on, thereby reducing the durability. 
     In some embodiments, the automobile transmission oil composition may further contain one or more additives which have conventionally been used in transmission oil compositions. The additives may make up the balance of the transmission oil based on the amounts of PAO synthetic base oil and viscosity index improver. For example, in some embodiments, one or more additives may be added in an amount of about 8 to 12 wt % with respect to the total weight of the manual transmission oil composition. Suitable additives include, but are not limited to, anti-wear agents, metallic detergent-dispersants, friction modifiers (particularly a phosphate ester friction modifier), dispersants (particularly a bis-succinimide ashless dispersant), and extreme pressure additives. As an anti-wear agent, any anti-wear agents that are known for inclusion in transmission oils can be used such as, for example, zinc dithiophosphate. Likewise, as a metallic detergent-dispersant, any materials that are known for inclusion in transmission oils can be suitably be used, such as, for example, calcium sulfonate. Extreme pressure additives can also be selected from any of those materials known for inclusion in transmission oils, such as, for example. polysulfide. 
     In another aspect, as discussed above, the present invention provides a method for lubricating automobile transmissions using the above-described lubrication oil compositions. As discussed above, the kind and size of the transmissions are not limited and, in certain embodiments, the transmission is a manual transmission. The above-described lubricating oil compositions may be supplied to the transmissions in any way known to those of skill in the art. 
     Next, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples. 
     Examples 1 To 3 &amp; Comparative Examples 1 to 6 
     Preparation of Manual Transmission Oil Compositions 
     Manual transmission oil compositions were prepared by stirring the components listed in Table 1 together at 60 to 80° C. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Classification 
                 Example 
                 Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 (Unit: wt %) 
                 1 
                 2 
                 3 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Synthetic base oil  (1)   
                 18.0 
                 18.0 
                 18.0 
                 41.0 
                 20.0 
                 37.0 
                 17.0 
                 35.0 
                 15.0 
               
               
                 Synthetic base oil  (2)   
                 61.0 
                 61.0 
                 61.0 
                 40.0 
                 61.0 
                 40.0 
                 60.0 
                 40.0 
                 60.0 
               
               
                 Viscosity index improver  (3)   
                 11.0 
                 11.0 
                 11.0 
                 9.0 
                 9.0 
                 13.0 
                 13.0 
                 15.0 
                 15.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Additives 
                 Dithiophosphate 
                 1.0 
                 1.0 
                 2.0 
                 1.0 
                 1.0 
                 2.0 
                 1.0 
                 1.0 
                 2.0 
               
               
                   
                 Calcium sulfonate 
                 4.0 
                 2.0 
                 3.0 
                 4.0 
                 2.0 
                 3.0 
                 4.0 
                 2.0 
                 3.0 
               
               
                   
                 phosphate ester 
                 2.0 
                 4.0 
                 2.0 
                 2.0 
                 4.0 
                 2.0 
                 2.0 
                 4.0 
                 2.0 
               
               
                   
                 Bis-succinimide 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
                 2.0 
               
               
                   
                 Polysulfide 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                   
                 Total 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
                 100.0 
               
               
                   
               
               
                   (1)  Chevron Phillips, PAO 4 (Kinematic viscosity at 100° C. 3.8 cSt; Viscosity index 130; Pour point −69° C. 
               
               
                   (2)  Chevron Phillips, PAO 6 (Kinematic viscosity at 100° C. 5.9 cSt; Viscosity index 135; Pour point −61° C. 
               
               
                   (3)  Lubrizol, Anglamol (Number-average molecular weight about 350,000) 
               
            
           
         
       
     
     Test Example: Evaluation of Viscosity (Kinematic Viscosity And Low-Temperature Viscosity At −40° C.) And Shear Stability 
     The kinematic viscosity (ASTM D 445) and low-temperature viscosity (ASTM D 2983) of the manual transmission oil compositions prepared in Examples 1 to 3 and Comparative Examples 1 to 6 were evaluated, and the results are shown in the following table 2. 
     The shear stability was evaluated for 48 hours according to CEC L-45-A-99 Viscosity shear stability of transmission lubricants (Taper Roller Bearing Rig), and the change in viscosity is shown as percentage in the following table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Example 
                 Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Classification 
                 1 
                 2 
                 3 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Kinematic 
                 at 100° C. 
                 11.2 
                 11.3 
                 11.2 
                 8.0 
                 8.5 
                 12.5 
                 13.0 
                 13.5 
                 13.5 
               
               
                 Viscosity 
                 at 40° C. 
                 51.1 
                 49.3 
                 49.5 
                 39.7 
                 42.1 
                 59.3 
                 61.2 
                 68.2 
                 69.2 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Viscosity index 
                 219 
                 228 
                 227 
                 179 
                 185 
                 215 
                 219 
                 206 
                 202 
               
               
                 Low-tem. viscosity 
                 11,000 
                 10,000 
                 10,000 
                 10,000 
                 10,500 
                 15,000 
                 18,500 
                 28,000 
                 30,000 
               
               
                 (at −40° C., cP) 
               
               
                 Shear stability (%) 
                 28 
                 28 
                 28 
                 28 
                 28 
                 32 
                 32 
                 35 
                 35 
               
               
                   
               
            
           
         
       
     
     As demonstrated in Table 1, while the shear stability and low-temperature viscosity of the oil compositions prepared in Comparative Examples 1 and 2 are good, the kinematic, viscosity is low, which is disadvantageous in terms of durability and rattle noise. Moreover, while the kinematic viscosity at a high temperature of the oil compositions prepared in Comparative Examples 3 to 6 is high, which is advantageous in terms of durability and rattle noise, the shear stability and low- temperature are notably lower than those of Examples 1 to 3. The kinematic viscosity at a high temperature of the oil compositions prepared in Example 1 to 3 was 11.2 to 11.3, which satisfies the requirements of transmission durability and reduced rattle noise. Furthermore, the low-temperature viscosity of the oil compositions prepared in Example 1 to 3 was 10,000 to 11,000, which exhibits excellent fluidity at low temperatures. Therefore, it is expected that the manual transmission oil compositions prepared in Examples 1 to 3 can improve the low- temperature shifting performance, and thus it can be seen that the oil compositions of Examples 1 to 3 have excellent viscosity characteristics compared to those of Comparative Examples 1 to 6. 
     As described above, the manual transmission oil composition according to the present invention improves the durability of the manual transmission, reduces the rattle noise, and improves the low-temperature shifting performance. 
     The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes 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.