Abstract:
Disclosed is an improved method of reducing compressor gas leakage by providing a compression cylinder with a lubricant comprising less than about 1% of a synergistic mixture of antioxidants.

Description:
RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 60/091,307, filed Jun. 30, 1998, which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method of reducing compressor gas leakage by providing a compression cylinder with a lubricant comprising less than about 1% of a synergistic mixture of antioxidants. 
     BACKGROUND ART 
     The escalating requirements for lubricants used in compression cylinders associated with the dynamic sealing of ethylene within the compressor cylinder and the hydraulic fatigue of pressure containing components subjected to cyclic pressures is considered one of the most demanding aspects of high-pressure manufacture of polyethylene. These aspects are discussed in more detail in  Lubrication Engineering , Volume 37, 4, 203-208 (1981). High demands for large scale production require increasingly improved lubricants. This objective becomes particularly difficult to achieve given the sophisticated and capital-intensive nature of the process and challenges the limitations of polyethylene manufacture. Thus, the combined requirements of high-pressure equipment and cylinder life pose a challenge which, to date, has not been satisfactorily achieved. 
     U.S. Pat. No. 4,654,154 to Wilkelski discloses a conventional method of reducing cylinder gas leakage from compressors used in high pressure olefin polymerization processes wherein from about 3 to about 10 wt % of an antioxidant is added to the lubricating fluid provided to a compressor cylinder. Wilkelski discloses that the problem of increased gas leakage is due to an oxidation reaction occurring in the narrow annular passageway between the plunger and the cylinder packing. 
     The oxidation can take place due to the presence of unreacted peroxide initiators in the recycled ethylene and/or the introduction of a polymerization initiator, that is, oxygen gas to the ethylene before compression. The unwanted oxidation leads to undesirable consequences including buildup of excessive heat, high pressures and resultant high mechanical stress. 
     Despite the benefits achieved by the Wilkelski method, the provision of the additional antioxidant forces the cost of practicing the method upwards, thereby adversely affecting the commercial viability of the process. 
     There exists a need to provide an efficient and cost-effective technique to satisfy the demands of high-pressure polyolefin manufacture. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention is a method of reducing compressor gas leakage comprising the step of: providing a compression cylinder with a lubricant comprising less than about 1.0 wt % of an antioxidant. 
     Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and obtained as particularly pointed out in the appended claims. 
     According to the present invention, the foregoing and other objects are achieved in part by a composition comprising a lubricant, and at least two antioxidants, wherein the antioxidants are present in an amount less than about 1 wt %, based on the total weight of the composition, and wherein the composition reduces cylinder gas leakage from a compressor used in high pressure olefin polymerization and increases time intervals between replacement of worn cylinder assemblies. 
     Additional objects and advantages of the present invention will become readily apparent to those having ordinary skill in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 
     DESCRIPTION OF THE INVENTION 
     The present invention addresses and solves problems stemming from the use of unnecessarily high amounts of costly antioxidants in connection with lubricating compression cylinders used in high pressure olefin polymerization processes. 
     In particular, the present invention is based, in part, on the surprising and unexpected discovery that equal or improved lubricant stability may be achieved at much lower levels of antioxidant through the use of synergistic mixtures of antioxidants comprising different blends of phosphite antioxidants, hindered phenols, and secondary amines. Numerous practical advantages in handling the smaller volume of solids stem from the present invention, and include less costly equipment for handling and less time required for batch preparation. 
     The lubricant component of the present invention may be any commercially available compound known to perform as a cylinder lubricant in a high pressure compressor for providing olefin feed to a polymerization zone. Examples include mineral oils and especially white mineral oils of 250 to 1200 SUS viscosity. The white mineral oils may also be combined with a polymeric thickener such as polybutene to adjust the viscosity to the desired level. 
     The antioxidant component of the present invention may be any antioxidants known to those of ordinary skill in the art. Examples include antioxidants comprising phosphite compounds and aminic and phenolic antioxidants. 
     An antioxidant of the present invention may be a phosphite antioxidant, for example, a phosphite antioxidant selected from the group consisting of trinonylphenyl phosphite, tris-tert-butylphenyl phosphite, tridecylphosphite, triphenylphosphite, trioctylphosphite, alkylphenylphosphites, and dilaurylphosphite. Preferred phosphite antioxidants include tris(2,4-di-tert-alkylphenyl)phosphite, more preferably tris(2,4-di-tert-butylphenyl)phosphite. The term “alkyl” is used throughout the present specification to mean an unbranched or a branched alkyl chain having from 1 to 8 carbon atoms. The amount of phosphite antioxidant characteristically present in the composition are critical high oxidation induction values and the total amount of antioxidant present does not exceed about 1 wt %, based on the total weight of the composition. A preferred tris(2,4-di-tert-butylphenyl)phosphite is Irgafos 168 sold commercially by Ciba-Geigy under that name. 
     Another example of an oxidant which may be useful in the present invention is a phenolic antioxidant. Preferred phenolic antioxidants are hindered phenols such as thiodiethylene bis(3,5-di-tert-alkyl-4-hydroxyhydrocinnamates, more preferably thiodiethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate which is sold commercially under the name Irganox 1035 by Ciba-Geigy, and tetrakis[methylene(3,5-di-tert-alkyl-4-hydroxyhydrocinnamate)]methanes, more preferably tetrakis[methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)] methane is sold commercially under the name Irganox 1010 by Ciba-Geigy. Another example of a hindered phenol useful in the present invention is butylated hydroxytoluene (BHT). The amount of phenolic antioxidant characteristically present in the composition is not particularly critical so long as the total amount of antioxidant present does not exceed about 1 wt %, based on the total weight of the composition. 
     A further example of an oxidant which may be useful in the present invention is an aminic antioxidant. Preferred aminic antioxidants include secondary aromatic amines such as diarylamines, e.g., diphenylamine, and modifieddiarylamines, e.g., N-phenyl-g-naphthylamine, p-isopropoxydiphenylamine, mono and dioctyldiphenylamine, bis-diarylamines and modified bisdiarylamines, such as N,N-diphenyl-p-phenyldiamine. The amount of aminic antioxidant characteristically present in the composition is not particularly critical so long as the total amount of antioxidant present does not exceed about 1 wt %, based on the total weight of the composition. 
     An especially preferred antioxidant is a blend of aminic and phenolic antioxidants, that is, a liquid blend of thiodiethylene bis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate, tetrakis[methylene(3,5-di-tert-but-hydroxyhydrocinnamate)] methane and alkylated diarylamines from the reaction products of 2,4,4-trimethylpentene and N-phenylbenzenamine, Irganox L 150, sold commercially by Ciba-Geigy under that name. 
     The antioxidant additives are present in the lubricant composition in an amount less than about 1 wt %, based on the total weight of the composition. Characteristically, the antioxidant additives are present in a range of about 0.4 to about 1.2 wt percent, preferably about 0.5 to about 1.1 wt percent, most preferably about 0.5 to about 1.0 wt percent. 
     The compositions of the present invention may further comprise other conventional lubricant additives, for example, thickeners such as polybutene Indopol H-1900, antiwear additives such as oleic acid and AMP, an amine phosphate wear additive, Irgalube 349 sold commercially by Ciba-Geigy under that name. 
     While the present compositions are disclosed generally above, additional embodiments are further discussed and illustrated with reference to the examples below. However, the examples are presented merely to illustrate the invention and are not considered as limitations thereto. 
    
    
     EXAMPLE 1 
     To demonstrate the efficacy of the antioxidant compositions according to the present invention, the oxidation stability was evaluated by pressure differential scanning calorimetry (DSC) at 175° C. and 500 psi oxygen. The results are shown in Table 1. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Compressor Lubricant Oxidation Stability by DSC 
               
               
                 Differential Scanning Calorimetry (DSC) at 175° C. and 500 psi Oxygen 
               
             
          
           
               
                   
                   
                 Oxidation Induct. 
               
               
                   
                   
                 Time, 
               
               
                 No. 
                 Lubricant Viscosity a)  and Added Antioxidants 
                 Min. @ 175° C. 
               
               
                   
               
               
                 1 
                 800 SUS Lubricant + 3.0% Oleic Acid b)  + 0.15% L-150 c)   
                 4.4 
               
               
                 2 
                 800 SUS Lubricant + 0.5% AMP d)  + 0.15% L-150 
                 5.8 
               
               
                 3 
                 800 SUS Base White Oil + Indopol H-1900 Thickener a)   
                 6.4 
               
               
                 4 
                 400 SUS Lubricant + 0.5% AMP + 0.15% L-150 
                 6.6 
               
               
                 5 
                 600 SUS Lubricant + 3% Oleic Acid b)  + 750 ppm BHT c)   
                 7.5 
               
               
                 6 
                 100 SUS Base White Oil + Indopol H-1900 Thickener 
                 7.5 
               
               
                 7 
                 1000 SUS Lubricant + 3% Oleic Acid + 750 ppm BHT 
                 7.5 
               
               
                 8 
                 540 SUS Base White Oil 
                 8.2 
               
               
                 9 
                 1200 SUS Lubricant + 0.5% AMP d)  + 1000 ppm BHT 
                 9.8 
               
               
                 10 
                 1000 SUS Lubricant + 0.5% AMP + 750 ppm BHT 
                 9.8 
               
               
                 11 
                 800 SUS Oil + 0.5% AMP d)  + 0.15% Irganox 1035 f)  + 0.4% Irgafos 
                 16.4 
               
               
                   
                 168 g)   
               
               
                 12 
                 800 SUS Lubricant + 0.5% AMP + 0.14% Irganox 1010 h)  + 0.4% 
                 18.6 
               
               
                   
                 Irgafos 168 
               
               
                 13 
                 600 SUS Lubricant + 3% Oleic Acid + 4% BHT 
                 20.8 
               
               
                 14 
                 800 SUS Lubricant + 0.5% AMP + 0.14% Irganox 1010 + 0.4% 
                 21.0 
               
               
                   
                 Irgafos 168 
               
               
                 15 
                 1500 800 SUS Lubricant + 0.5% AMP + 0.5% Irganox 1035 
                 21.3 
               
               
                 16 
                 800 SUS Lubricant + 0.5% AMP d)  0.8% L-150 + 0.24% Irgafos 168 
                 21.4 
               
               
                 17 
                 600 SUS Libricant + 3% Oleic Acid b)  + 0.15% Irganox L-150 + 0.5% 
                 22.7 
               
               
                   
                 Irgafos 168 
               
               
                 18 
                 400 SUS Lubricant + 0.50% AMP d)  + 0.08% Irganox L-150 + 0.24% 
                 31.1 
               
               
                   
                 Irgafos 168 
               
               
                 19 
                 800 SUS Lubricant + 0.5% AMP d)  + 0.15% Irganox L-150 + 0.40% 
                 35.0 
               
               
                   
                 Irgafos 168 
               
               
                 20 
                 1500 SUS Lubricant + 0.5% AMP d)  + 0.33% Irganox L-150 
                 37.0 
               
               
                 21 
                 800 SUS Lubricant + 0.5% AMP d)  + 0.15% L-150 + 0.5% Irgafos 168 
                 50.4 
               
               
                 22 
                 600 SUS Lubricant + 0.5% AMP d)  + 0.15% L-150 + 0.5% Irgafos 
                 54.0 
               
               
                   
                 168 f)   
               
               
                 23 
                 1000 SUS Lubricant + 0.5% AMP d)  + 0.15% L-150 + 0.5% Irgafos 
                 53.5 
               
               
                   
                 168 
               
               
                 24 
                 1200 SUS Lubricant + 0.5% AMP d)  + 0.33% Irganox L-150 + 0.5% 
                 65.0 
               
               
                   
                 Irgafos 168 
               
               
                   
               
               
                   a)  Oil viscosities are SUS at 100° F. All lubricants contain USP White oils as base with polybutene Indopol H-1900 as thickener.  
               
               
                   b)  Oleic acid is antiwear additive.  
               
               
                   c)  Irganox L-150, an antioxidant liquid blend from Ciba consisting of Irganox L-57 secondary amine antioxidant plus Irganox 1010, and Irganox 1035, hindered phenolic antioxidants.  
               
               
                   d)  AMP is Irgalube 349, an amine phosphate wear additive from Ciba  
               
               
                   e)  BHT is butylated hyroxytoluene, hindered phenolic antioxidant  
               
               
                   f)  Irganox 1035, hindered phenolic antioxidant from Ciba  
               
               
                   g)  Irgafos 168, aryl phosphite antioxidant from Ciba  
               
               
                   h)  Irganox 1010, hindered phenolic antioxidant from Ciba  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Block-On-Ring Wear Test Results for Oxidation Inhibitor Oils 
               
             
          
           
               
                   
                 780-48-2 
                 780-48-3 
                 780-49-1 
                 780-49-2 
                 780-49-3 
                 Run 115 
                 Run 113 
                 715-89-600 
                 715-89-1000 
               
               
                   
                   
               
             
          
           
               
                 Vis SUS @ 100° F. 
                 800 
                 400 
                 800 
                 400 
                 800 
                 1000 
                 1000 
                 600 
                 1000 
               
               
                 Identity in Table 1 
                 No. 16 
                 No. 18 
                 No. 2 
                 No. 4 
                 No. 1 
                 No. 10 
                 No. 7 
                 No. 22 
                 No. 23 
               
               
                 BHT, ppm 
                   
                   
                   
                   
                   
                  750 
                  750 
               
               
                 Irgalube 349 
                 0.50 
                 0.50 
                 0.50 
                 0.50 
                 — 
                 0.5  
                 — 
                 0.50 
                 0.5  
               
               
                 Irganox L-150 
                 0.08 
                 0.08 
                 0.15 
                 0.15 
                 0.15 
                 — 
                 — 
                 0.15 
                 0.15 
               
               
                 Irgafos 168 
                 0.24 
                 0.24 
                 — 
                 — 
                 — 
                 — 
                 — 
                 0.50 
                 0.50 
               
               
                 Oleic Acid 
                 — 
                 — 
                 — 
                 — 
                 3.0  
                 — 
                 3.0  
                 — 
                 — 
               
               
                 Block-on-Ring a)   
               
               
                 Scar, mm 
                 0.70 
                 0.60 
                 0.70 
                 0.70 
                 0.70 
                 0.80 
                 0.95 
                 0.60 
                 0.60 
               
               
                 μ, friction coefficient 
                  0.037 
                  0.037 
                  0.036 
                  0.038 
                 0.04 
                 0.03 
                 0.02 
                 0.03 
                 0.04 
               
               
                 Block w/loss, mg 
                 0.3  
                 0.2  
                 0.1  
                 0.2  
                 0.2  
                 0.01 
                 1.15 
                 0.00 
                 0.1  
               
               
                 DSC @ 175° C., Min. 
                 21.4  
                 31.1  
                 5.8  
                 6.6  
                 4.4  
                 9.8  
                 7.5  
                  54 
                 53.5  
               
               
                 Oxidation Induction 
               
               
                 Time b)   
               
               
                   
               
               
                   a) ASTM G77, Modified Test Conditions: 210° F., 750 rpm, 200 lb load, 20,000 cycles; 2 min break-in at 30 lb load Kennametal KZ 94 tungsten carbide rings with 1 microinch average roughness. CDA copper alloy No. C93800 bronze block.  
               
               
                   b) Differential Scanning Calorimetry (DSC) at 175° C. and 500 psi oxygen; general procedure described in ASTM D-5483.  
               
             
          
         
       
     
     Table 2 is a summary of matching metallurgy wear tests with corresponding DSC data from Table 1. Cross references are given in Table 2 for referring back to Table 1. A word about the importance of using matching metallurgy in the bench wear test: the sealing elements in the compressors are the polished Kenemetal KZ 94 tungsten carbide plungers and special bronze packing rings. We used CDA 938 bronze because this type of bronze is the choice of several polyethylene manufacturers. Both the elements (rings and bronze blocks) were obtained from the industry suppliers of plungers and bronze seal rings. 
     It should be noted that formulations using the prior art give a DSC oxidation time of 20.8 minutes, see example 13 in Table 1. By adding phosphite and other antioxidants, oxidation inhibition is enhanced for all formulations containing the L-150 antioxidant blend and Irgafos 168 (at much lower total antioxidant levels). 
     Especially dramatic are the results when one compares the oxidation induction times of the following lubricants in Table 1: #2 and #4 versus #22 and #23. Only 0.5 wt % Irgafos 168 improves the oxidation stability by almost a factor of 10. In example 24 (0.83% total antioxidant) of Table 1, the oxidation induction time is 65 minutes compared to reference example #13 where the BHT total antioxidant level is 4%. 
     In Table 2, we note that better wear results were obtained with the new antioxidant packages. 
     Only the preferred embodiment of the invention and an example of its versatility is shown and described in the present disclosure. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.