Synthetic bearing lubricant
The invention concerns a synthetic lubricant for bearings, in particular for highly stressed bearings such as the backing roll bearings of rolling mills. 
     Metallic foil is produced mostly by cold rolling on a quarto rolling mill. In this rolling process the rolling force is transferred to the two rolls via the upper and lower backing rolls. The resultant large forces are transmitted via the roll axles of the backing rolls to the roll bearings and then to the rolling mill stands. The tribological events which occur during the lubrication of loaded, rotating roll bearings can be described in a simplified manner as follows: 
     The roll axle made of hard roll steel is embedded in a ring made of soft bearing metal. The bearing with the sealing rings closed towards the roll side is flushed with a lubricant. This facility produces a film of lubricant between the roll axles and the bearing rings. The film of lubricant separates the rotating axles from the bearing rings, prevents contact between the two metal surfaces and transmits the rolling force from the rolls to the stands. 
     On starting up or slowing down, or when heavy forces are developed in the roll bearings the lubricant film is partly broken with the result that the metallic surfaces of the axles and roll bearings come into contact. In this condition of mixed friction/lubrication the specific extreme pressure additives in the lubricant ensure that no &#34;cold welding&#34; occurs between the two metal surfaces. These additives--specific organic substances dissolved in lubricant oil--react with the metal surface which has been made highly active as a result of the rubbing contact, and produce a reaction layer which prevents metal contact and thus wear on the bearings. 
     When extremely high loading occurs and the lubricant can no longer prevent direct contact between the metal surfaces, rapid wear of the bearing surfaces occurs due to high frictional forces, metallic fines and the large amount of heat developed. Consequently high temperatures are produced, at which the bearing metal becomes liquid and the self-ignition temperature of the lubricant can be reached. 
     In foil rolling mills for example the bearing lubricant can be heavy oil derivatives containing lead naphthenic salts as an additive. Leaks in the bearing seals can cause the foil rolling lubricant--normally with palmseed oil (a mixture of natural tri-glycerides) as the pressure component--to be contaminated with roll bearing lubricant. If the concentration of roll bearing lubricant in the foil rolling oil exceeds a certain level, then after the degreasing heat treatment of the rolls of foil, sticky foil will result. When thin strip is given a degreasing heat treatment, then the bearing lubricant in the rolling oil causes brown spots and edges on the surface of the rolled product. 
    
    
     With this in mind the inventor set himself the task of developing a synthetic roll bearing lubricant with which the wear on the roll bearings can be avoided even under very high loads, whereby even large amounts of impurities of the bearing lubricant in the rolling oil do not cause spots on the rolled product after the degreasing heat treatment. 
     The bearing lubricant of the invention is such that 100 parts by weight of the lubricant contain: 
     
         ______________________________________                                    
86.9 to 97.5                                                              
        parts by weight of polyisobutene                                  
 2.1 to 11.2                                                              
        parts by weight of at least one ester of a C.sub.2                
        to C.sub.5 alcohol with an α-hydroxy-monocarboxylic         
        acid or an α-hydroxy-dicarboxylic acid, and                 
 0.4 to parts by weight of at least one single or                         
        multiple-unsaturated C.sub.13 -to C.sub.19 -carboxylic            
        acid.                                                             
______________________________________                                    
 
    
     Preferably the 100 parts by weight of the lubricant comprises: 
     
         ______________________________________                                    
91.5 to 95.3                                                              
        parts by weight of polyisobutene                                  
 3.7 to parts by weight of at least one ester of a C.sub.2                
        to C.sub.5 alcohol with an α-hydroxy-carboxylic             
        acid, and                                                         
 0.6 to parts by weight of at least one unsaturated                       
        C.sub.13 -C.sub.19 carboxylic acid.                               
______________________________________                                    
 
    
     The manufacture of the lubricant takes place simply by mixing the components in accordance with the required composition. The mixing operation can be made easier by warming the viscous components. 
     Extensive plant trials with various lubricating systems have shown that the lubricant of the invention can be employed to full advantage in all known lubricating systems (e.g. closed circuit systems and various open circuit systems with ball, roller and cone bearings). It has been found particularly advantageous to employ the lubricant of the invention in oil mist and oil droplet/compressed air lubricating systems. It was found, surprisingly, that the friction which is found to occur with ring bearings on starting up cold, can be avoided to a large extent with the lubricant according to the invention. Trials have shown that the tendency of the rolls to stick can be reduced without impairing the advantages accrued from the invention, if the polyisobutene is completely or partly replaced by polymethyacrylate dissolved in mineral oil, or by a mixture of polymethacrylate and kerosene dissolved in mineral oil. In the latter case the ratio of polymethacrylate solution: kerosene should be approx. 2:1 to 1:2, preferably 1.2:1 to 1:1.2. The kind of mineral oils containing polymethacrylate are commercially available products (e.g. VISCOPLEX SV 36, Rohm GmbH), and are in general used as lubricant additives to lower the stock point and to raise the viscosity index. 
     The polybutenes used in accordance with the invention to form the hydrodynamic lubricant film are likewise commercially available products e.g. INDOPOL L 10 and INDOPOL H 100 (AMOCO CHEMICALS). The viscosity of the lubricant of the invention can be altered over a relatively large range simply by mixing in polybutenes of various chain lengths. For example by altering the ratio of mixing of the two above mentioned INDOPOLES, at 60° C. a viscosity range of 10 to 5000 cSt can be obtained. 
     Direct contact between the roll axles and the roll bearings is prevented mainly by the hydrodynamic lubrication effect of the polybutene based lubricant of the invention. This hydrodynamic lubricant film can be broken by heavy loads with the result that wear occurs and in an extreme case could cause cold welding between the axles and the bearings. This can be prevented by the addition, in accordance with the invention, of α-hydroxymonocarboxylic acid esters, which are provided to form a &#34;reaction layer&#34; via tribo-chemical reactions when the hydrodynamic lubricant film is broken. The lubricant additives chosen for this role are therefore denoted as &#34;reaction layer agents&#34;. 
     It has been found that the preferred esters are those whose alcohol component is an alcohol from the following group: 
     ethanol 
     propanol 
     butanol 
     pentanol 
     and the acid component is an acid from the following group: 
     hydroxy acetic acid 
     lactic acid 
     malic acid 
     tartaric acid 
     The following of these possible combinations have been found to be particularly advantageous: 
     ethanol hydroxy acetate 
     propanol hydroxy acetate 
     butanol hydroxy acetate 
     lactic acid ethyl ester 
     lactic acid propyl ester 
     lactic acid butyl ester 
     malic acid butyl ester 
     tartaric acid dibutyl ester 
     3-hydroxy-butyric acid ethyl ester 
     Furthermore it has been found that the rust inhibiting single or poly unsaturated C 13  to C 19  carboxylic acid is advantageously of the group: 
     oleic acid 
     linoleic acid 
     linolenic acid. 
     In order to achieve the optimum properties in the lubricant according to the invention it has been found favorable for the individual components, in particular the esters which form the reaction layer, to be at least of &#34;pure&#34; grade. 
     The advantages of the bearing lubricant in accordance with the invention will now be explained further with the help of results from a number of investigations. 
     The compositions of the exemplified embodiments of the lubricants in accordance with the invention, their densities and viscosities are given in table I. 
     
                                           Table I                                 
__________________________________________________________________________
                   Concentration Kinematic viscosity (cSt)                
Lubricant                                                                 
      Composition  (parts by weight)                                      
                            Density                                       
                                 20° C.                            
                                     50° C.                        
                                         60° C.                    
                                             80° C.                
__________________________________________________________________________
4661  INDOPOL H 100                                                       
                   40       0.860                                         
                                 273 50  33  17                           
      INDOPOL L 10 60                                                     
      butanol hydroxy acetate                                             
                   6                                                      
      Oleic acid   1                                                      
6461  INDOPOL H 100                                                       
                   60       0.869                                         
                                 1098                                     
                                     138 86  39                           
      INDOPOL L 10 40                                                     
      butanol hydroxy acetate                                             
                   6                                                      
      Oleic acid   1                                                      
4662  INDOPOL H 100                                                       
                   40       0.860                                         
                                 346 62  51  20                           
      INDOPOL L 10 60                                                     
      lactic acid butylester                                              
                   6                                                      
      oleic acid   1                                                      
4663  INDOPOL H 100                                                       
                   40       0.860                                         
                                 466 77  49  24                           
      INDOPOL L 10 60                                                     
      malic acid butylester                                               
                   6                                                      
      oleic acid   1                                                      
4664  INDOPOL H 100                                                       
                   40       0.864                                         
                                 561 85  53  25                           
      INDOPOL L 10 60                                                     
      tartaric acid butylester                                            
                   6                                                      
      oleic acid   1                                                      
4665  INDOPOL H 100                                                       
                   40       0.858                                         
                                 405 62  40  20                           
      INDOPOL L 10 60                                                     
      3 hydroxybutyric acid                                               
      ethylester   6                                                      
      oleic acid   1                                                      
4666  INDOPOL H 100                                                       
                   40       (1)  709 100 61  27                           
      INDOPOL L 10 60                                                     
      tartaric acid diethylester                                          
                   6                                                      
      oleic acid   1                                                      
556 V VISCOPLEX SV 36                                                     
                   35       0.867                                         
                                 707 231 174 109                          
      butanol hydroxyacetate                                              
                   5                                                      
      oleic acid   1                                                      
      petrol       38                                                     
__________________________________________________________________________
 (1) Tartaric acid-diethylester is only partially soluble in polyisobutene
 
    
     After the last rolling pass, metallic foils are given a heat treatment partly with the aim of removing residual rolling lubricant from the surface. This heat treatment is carried out either in a furnace with air circulating in it or under nitrogen as a protective atmosphere. Since the rolling lubricant can be contaminated with bearing lubricant during production, as a result of leaks in the roll bearings, the behavior of this lubricant during the heat treatment is of decisive importance, in particular in the production of aluminum foil. 
     In table II results from tests closely related to actual practice are presented. For comparison purposes conventional lubricants of the following kinds were included in the tests: 
     Commercially available product A: 
     Commercially available product B: oil-mist-lubricant 
     Commercially available product C: gear lubricating oil 
     Commercially available product D: gear lubricating oil. 
     Heat treatment test No. 1 is a test in which a drop of lubricant is placed between two pieces of aluminum foil to simulate the conditions under which the foil is heat treated. After annealing at 400° C. in a furnace with air circulating in it, the force of adhesion between the pieces of foil, the size of a drop of water on the foil surface and the degree of staining determined visually. The following classification system is used to determine the degree of staining: 
     4=pronounced staining 
     3=staining 
     2=mild staining 
     1=recognizable discoloring due to residues 
     0=no discoloring 
     To carry out heat treatment test No 2 a drop of lubricant is placed on a foil in which a recess has been made, and the foil then heat treated at 400° C. This test is always carried out on a pair of such samples, one sample being heat treated in a furnace with air circulating in it, the other in a nitrogen atmosphere. 
     The results of these trials are also presented in table II, the assessment of the degree of staining being the same as in heat treatment test No. 1. 
     
                                           Table II                                
__________________________________________________________________________
               Heat treatment test No. 1                                  
                                 Heat treatment test No. 2                
               adhesive force                                             
                       drop test Staining                                 
                                        Staining                          
Lubricant      g/44.4 mm                                                  
                       φ mm                                           
                            Staining                                      
                                 in air (in N.sub.2)                      
__________________________________________________________________________
Palmseed oil   &gt;30     --   4    4      4                                 
10% Palmseed oil in Kerosene                                              
               3.5     7    0    0      3                                 
Product A      34      --   4    4      4                                 
10% Product A in Kerosene                                                 
               0.2     7    0    0      4                                 
Product B      9.4     4.5  3    4      0                                 
10% Product B in Kerosene                                                 
               2.8     7    0    1      0                                 
Cylinder oil   &gt;30     --   4    4      1                                 
10% Cylinder oil in Kerosene                                              
               3.5     6    0    0      1                                 
Product C      0.1     12   0    3      1                                 
10% Product C in Kerosene                                                 
               0.1     12   0    0      0                                 
Product D      &lt;0.1    12   0    4      1                                 
10% Product D in Kerosene                                                 
               &lt;0.1    11   0    0      0                                 
INDOPOL H 100  0       12   0    3      0                                 
10% INDOPOL in Kerosene                                                   
               0       12   0    0      0                                 
6461           0.02    13   0    1      0                                 
10% 6461 in Kerosene                                                      
               0.02    13   0    0      0                                 
4661           0.02    15   0    2      0                                 
10% 4661 in Kerosene                                                      
               0.02    15   0    1      0                                 
4662           0.02    12   0    1      0                                 
10% 4662 in Kerosene                                                      
               0.02    11   0    1      0                                 
4663           0.02    12   0    3      0-1                               
10% 4663 in Kerosene                                                      
               0.02    11   0    3      0-1                               
4664           0.02    12   0    4      0-1                               
10% 4664 in Kerosene                                                      
               0.02    11   0    4      0-1                               
4665           0.02    12   0    4      0-1                               
10% 4665 in Kerosene                                                      
               0.02    11   0    4      0-1                               
4666           0.02    11   0    2      0                                 
10% 4666 in Kerosene                                                      
               0.02    12   0    2      0                                 
556 V          2.6     15   0    4      1                                 
10% 556 V in Kerosene                                                     
               0.09    15   0    4      1                                 
Petrol         0       12   0    0      0                                 
__________________________________________________________________________
 
    
     The results of the wear test (RV-test) by Baist at 60° C. are given in table III. Steel pins, 1700 μm long and 5 mm in diameter were employed for this test. The area of pin under load was 12.6 mm 2 . 
     The column headed &#34;start of fines&#34; gives the load in kg/12.6 mm 2  at which the first steel fines were observed. In the column headed &#34;start of friction&#34;, the load at which the first rubbing occured. The limit was taken as the load at which the first &#34;cold weld spots&#34; appeared, at which stage the electrical conductivity of the film of lubricant had fallen to such a low value that it could be assumed that locally a complete break down in the lubricant film had occured. The column &#34;end of friction&#34; gives information about the point at which the limiting load is reached. 
     The comparison of the results from the lubricant of the invention and the commercially available product A shows clearly the superior performance of the lubricant of the invention in that the beakdown of the lubricant film does not occur until higher loads are reached. 
     
                       Table III                                                   
______________________________________                                    
start           end                                                       
        fines   friction                                                  
                        limiting load                                     
                                 resistance                               
                                        friction                          
Lubricant                                                                 
        (kg)    (kg)    (kg)     (Ω)                                
                                        (kg)                              
______________________________________                                    
Product A                                                                 
        140     40      230      1      25                                
6461    110     60      310      2      30                                
4661    50      150     540      0.5    20                                
4662    50      80      290      0      30                                
4663    100     100     290      0      30                                
4664    50      50      310      0.5    30                                
4665    90      90      380      0      30                                
4666    70      70      430      0.5    30                                
556 V   20      50      350      0      20                                
______________________________________