Abstract:
Visco-elastic tire balancing compositions comprising 1) 85 to 97% by weight of a glycol ether component comprising one or more ethylene/propylene glycol copolymer ethers of the general formula (I) or the general (II) or mixtures thereof R—O{[CH(CH 3 )CH 2 —O—] m [CH 2 —CH 2 —O—] n }H (I) R 1 —(O—{[CH(CH 3 )CH 2 —O—] m [CH 2 —CH 2 —O—] n }H) 2  (II) wherein R is hydrogen or an alkyl group of 2-8 carbon atoms; R 1  is an alkylene moiety of 2-8 carbon atoms in which the two substituents are not carried on the same carbon atom; m is the mole percentage of propylene glycol in the ethylene/propylene glycol copolymer moiety or moieties; and n is the mole percentage of ethylene glycol in the ethylene/propylene glycol copolymer moiety or moieties, wherein the ratio n:m is in the range from 35:65 to 80:20; each glycol copolymer compound having a number average molecular weight in the range of 2000-10000; and 2) 3 to 15% by weight of a fumed silica gel former; said balancing compositions being visco-elastic 1500 Pa and 5000 Pa at 22° C., a Loss Modulus (G″) smaller than the Storage Modulus up to a Cross Over Frequency of 10-40 Hz, and a Critical Yield Stress exceeding 2 Pa.

Description:
FIELD OF THE INVENTION 
     The present invention relates to improved visco-elastic, chemically inert gel compositions for use in eliminating imbalances in motor vehicle wheel assemblies, a method for preparation of such compositions as well as the use of such compositions. 
     BACKGROUND OF THE INVENTION 
     Vibrations in rolling wheel assemblies (i.e. tyre+rim) are caused by several, different types of tyre imperfections, the most important being
         Non-homogeneous mass distribution   Stiffness variations   Geometric variations   Radial and lateral run out   Eccentricity of the tyre       

     Similar imperfections in the rim will also induce vibrations. 
     Vibrations in a truck or car wheel assembly due to imperfect tyres or rims cause vibrations which, apart from the discomfort to the driver and any passengers, can dramatically increase the tyre wear as well as mechanical wear. The present most common method for reducing wheel assembly derived vibration is still the attachment of metal weights to the tyre rim. However, this balancing technique, being of a static nature, cannot compensate for changes in load (i.e. deformation of the tyres), uneven wear of the tyres and the consequent changes in mass distribution, dirt collection on the rim, and the like. Consequently, balancing by means of metal weights must be repeated several times during the life time of a tyre. 
     The present invention is based on the fact that vibrations caused by imbalance in a wheel assembly can induce a motion in a liquid inside the tyre in a direction that will diminish the vibrations and the imbalance. However, to utilize this “vibrational pressure” to achieve balancing of a wheel assembly (i.e. that the gravitational centre is in the intersection between the rotational plane and the axis of rotation) is by no means trivial. If a mere free-flowing liquid such as water was introduced into a spinning tyre, any movement of this mass caused by a “vibrational pressure” would immediately be counteracted by the centrifugal force, which frequently exceeds 1000×g during normal driving, resulting in an oscillating movement of the liquid with no balancing effect. 
     These oscillations can be prevented/dampened sufficiently to achieve a balancing effect by using visco-elastic gels characterized by their critical yield stress and other rheological parameters. 
     U.S. Pat. No. 5,540,767, the content of which is hereby incorporated by reference, discloses visco-elastic tyre balancing compositions comprising (A) 80-95% w/w of an oil selected from i.a. polypropyleneglycol alkyl ethers, and (B) 4-15% w/w of a gel former selected from i.a. fumed silica having a BET surface in the range of from about 50 to about 400 m 2 /g. 
     The efficiency of these balancing gels can be demonstrated experimentally by measuring vertical accelerations on the spring leg of a front axle with its wheels running under load against a rotating drum. 
     SUMMARY OF THE INVENTION 
     The invention relates to improved visco-elastic tyre balancing compositions comprising
         1) 85 to 97% by weight of a glycol ether component comprising one or more ethylene/propylene glycol copolymer ethers of the general formula (I) or the general (II) or mixtures thereof
 
R—O{[CH(CH 3 )CH 2 —O—] m [CH 2 —CH 2 —O—] n }H  (I)
 
R 1 —(O—{[CH(CH 3 )CH 2 —O—] m [CH 2 —CH 2 —O—] n }H) 2   (II)
   wherein   R is hydrogen or an alkyl group of 2-8 carbon atoms;   R 1  is an alkylene moiety of 2-8 carbon atoms in which the two substituents are not carried on the same carbon atom;   m is the mole percentage of propylene glycol in the ethylene/propylene glycol copolymer moiety or moieties; and   n is the mole percentage of ethylene glycol in the ethylene/propylene glycol copolymer moiety or moieties, wherein the ratio n:m is in the range from 35:65 to 80:20;   each glycol copolymer compound having a number average molecular weight in the range of 2000-10000;       

     and 
     2) 3 to 15% by weight of a fumed silica gel former; 
     said balancing composition being visco-elastic and having a Storage Modulus (G′) between 1500 Pa and 5000 Pa at 22° C., a Loss Modulus (G″) smaller than the Storage Modulus up to a Cross Over Frequency of 10-40 Hz, and a Critical Yield Stress exceeding 2 Pa. 
     Is has been found that compared with the pure polypropylene glycol alkyl ethers used in U.S. Pat. No. 5,540,767, the use according to the invention of ethylene/propylene glycol copolymer monoethers results in improved performance of tyre balancing compositions. 
     The invention further relates to an automobile tire containing within its air cavity a suitable amount of a balancing composition of the invention, and an automobile wheel assembly comprising a wheel rim and such a tire, as well as a method for balancing a motor vehicle wheel assembly, comprising applying to the inner surface of the tire a balancing composition according to the invention, mounting the wheel assembly on a motor vehicle and driving the motor vehicle for a distance sufficient to allow the balancing composition to balance the wheel assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-section of tyre depicting a preferred arrangement of a balancing gel of the invention therein. 
         FIG. 2  shows a plot in the linear visco-elastic region (oscillation freq. 1 Hz, stress sweep 0.1-1000 Pa) of the Storage Modulus G′ (in Pa) versus the stress (in Pa) for some balancing gel compositions of the invention described in Table 1. 
         FIG. 3  shows a plot in the linear visco-elastic region (oscillation freq. 1 Hz, stress sweep 0.1-1000 Pa) of the Phase Angle (in degrees (°)) versus the stress (in Pa) for some balancing gel compositions of the invention described in Table 1. 
         FIG. 4  shows a plot of the Storage Modulus G′ (in Pa) and the Loss Modulus G″ (in Pa) versus the frequency in a frequency sweep for some balancing gel compositions of the invention described in Table 1. Frequency sweep 100-0.1 Hz, 10 or 5 Pa stress (stress within Linear Visco-Elastic region). Arrows indicate crossover frequency. 
         FIG. 5  shows a plot of viscosity (in Pa.s) and strain against shear stress (in Pa) in connection with determination of yield stress. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Important rheological properties of the balancing composition are its Critical Yield Stress (CYS) and Elastic (Storage) Modulus, G′, both measured in the linear visco-elastic region, as well as its Yield Stress as determined in stress growth measurements and the relationship between its storage modulus, G,′ and its Loss Modulus, G″, measured by a frequency sweep. 
     G′ is a measure of the strength of the gel, that is the strength and the number of bonds between the molecules of the gel former. I have found that in order to achieve satisfactory balancing properties the value of G′ should be in the range 1500 to 5000 Pa and the Critical Yield Stress should be in the range 5-30 Pa. Stress sweep of a series of functional balancing gels is shown in  FIGS. 2 and 3 . A very satisfactory balancing composition has a value of G′ around 2000 Pa and a Critical Yield Stress around 18 Pa. 
     G″ is a measure of a material&#39;s ability to dissipate energy in the form of heat. 
     The relationship between G′ and G″ as measured in a frequency sweep is a structural Characterization of a material. Typical frequency sweep curves for a series of well working balancing compositions are shown in  FIG. 4 . The “Cross Over Frequency”, i.e. the frequency at which G″ becomes greater than G′ should be in the range 10-40 Hz. A very satisfactory balancing composition has a cross over frequency value around 16 Hz. Typical stress growth sweeps for two functional balancing compositions are shown in  FIG. 5 . A very satisfactory balancing gel has a Yield Stress of about 30 Pa. 
     Of equal importance as the visco-elastic properties is the long term stability in service of the balancing gel, the performance at various temperatures of the gel, and the chemical inertness of the gel. 
     An acceptable tyre balancing gel must remain functional during the life time of the tyre and under the various driving conditions, in particular it must remain functional within the normal driving speed interval, 0 to approximate 160 KMH, and within the normal temperature range, −30° C. to +90° C., experienced in service. 
     Furthermore, the tyre balancing gel must not have any harmful effect on the tyre, in particular it should not affect the inner lining of the tyre or cause any damage to the cord or other tyre parts if it accidentally reaches such parts (i.e. by puncture of the tyre). 
     Minimum Criteria for a Functional Balancing Gel 
     Rheology: 1500 Pa&lt;Storage Modulus (G′)&lt;5000 Pa. 
     Cross Over Frequency (G″&gt;G′) 10-40 Hz. 
     Critical yield Stress&gt;2 Pa. 
     Yield Stress&gt;5 Pa 
     Volatility: Less than 6% (by weight) evaporation loss after 10 hours at 99° C. 
     Pour point of base liquid: &lt;−15° C. (ASTM D97) 
     Separation stability: Less than 20% separation (by weight) of the base oil after 12 hours at 300 000×g and 25° C. 
     Chemical reactivity: No effect on rubber or other polymers, non-corrosive to metals. 
     As indicated above, the gels of the invention comprise two components, namely a base liquid and a gel former. 
     In the above formulas (I) and (II) for the base liquids, the square parentheses enclosing the propylene glycol and ethylene glycol moieties, respectively, are intended to denote that the monomer units of the copolymers may be present in the linear copolymer in any copolymer arrangement, such as random copolymers or block copolymers, random copolymers presently being preferred. 
     The copolymers of formula (I) are called alcohol initiated, and the copolymers of formula (II) are called diol initiated. The alkyl moiety R of the initiating alcohol may have 1-6 carbon atoms. Suitable examples are n-propanol, n-butanol, n-pentanol or n-hexanol. The alkylene moiety R 1  of the diol in question may be linear or branched and may have 2-8 carbons. Examples of the initiating diol may be ethylene glycol, propylene glycol, 1,3-propane diol, 1,3-butane diol, or 1,6-hexane diol. 
     It is preferred that the number average molecular weight of the glycol ether component(s) is/are in the range of 3000-10000. 
     In an embodiment of the invention, the ratio n:m of a copolymer of formula (I) is in the range from 40:60 to 75:26, such as from 40:60 to 60:40, in particular from 45:55 to 55:45, such as around 50:50. In another embodiment, the ratio n:m of a copolymer of formula (II) may be from 70:30 to 80:20, such as around 75:25. 
     In a preferred embodiment, the composition contains 6.3±0.2% w/w, more preferably 6.3±0.1%, of fumed silica gel former; and 1±0.3% w/w, preferably 1±0.2 w/w, more preferably 1±0.1% w/w of a copolymer of formula (II) having a ratio n:m from 70:30 to 80:20, more preferably around 75:25; and the balance being a copolymer of formula (I) having a ratio n:m from 45:55 to 55:45, such as around 50:50. 
     In another preferred embodiment, composition contains 6.5±0.2% w/w, more preferably 6.5±0.1%, of fumed silica gel former; and 1±0.3% w/w, preferably 1±0.2 w/w, more preferably 1±0.1% w/w of a copolymer of formula (II) having a ratio n:m from 70:30 to 80:20, more preferably around 75:25; and the balance being a copolymer of formula (II) having a ratio n:m from 45:55 to 55:45, such as around 50:50. 
     Examples of base liquids useful in the practice of the invention are: 
     UCON® 50-HB Fluids, which are alcohol-initiated random copolymers of ethylene oxide and propylene oxide containing equal amounts by weight of oxyethylene and oxypropylene groups. The 50-HB Series products are water soluble at temperatures below 40° C. and have one terminal hydroxyl group; 
     SYNALOX® 40 fluids (produced by Dow Chemical Company), which are water soluble, diol-initiated random copolymers of ethylene oxide and propylene oxide containing 40% by weight of oxyethylene and 60% by weight oxypropylene groups, and have two terminal hydroxyl groups; 
     SYNALOX® 50 fluids which are water soluble, diol-initiated random copolymers of ethylene oxide and propylene oxide containing 50% by weight of oxyethylene and 50% by weight oxypropylene groups, and have two terminal hydroxyl groups; and 
     UCON® 75-H Fluids which are diol-initiated random copolymers of ethylene oxide and propylene oxide containing 75 weight percent oxyethylene and 25 weight percent oxypropylene groups. 75-H Series products are water soluble at temperatures below 75° C. and have two terminal hydroxyl groups. 
     Specific examples of base liquids useful in this invention are 
     1) a butanol-initiated random copolymer of ethylene oxide and propylene oxide containing equal amounts by weight of oxyethylene and oxypropylene groups with a numbered average molecular weight of 3930, viscosity 1100 cSt at 40° C. and ISO 3448 viscosity grade of 1000 available from DOW Chemical Corporation under the trade name UCON® 50-HB-5100, 
     2) a diol-initiated random copolymer of ethylene oxide and propylene oxide containing 40% by weight of oxyethylene and 60% by weight oxypropylene groups with a numbered average molecular weight of 5300, viscosity 1050 cSt at 40° C. and ISO 3448 viscosity grade of 1000 available from DOW Chemical Corporation under the trade name SYNALOX® 40-D700, 
     3) a diol-initiated random copolymer of ethylene oxide and propylene oxide containing 50% by weight of oxyethylene and 50% by weight oxypropylene groups with a kinematic viscosity of 960-1.160 mm2/s@40° C. ASTM D445 available from DOW Chemical Corporation under the trade name SYNALOX®50-D700 and 
     4) a diol-initiated random copolymer of ethylene oxide and propylene oxide containing 75 weight percent oxyethylene and 25 weight percent oxypropylene groups with a numbered average molecular weight of 6950 and viscosity 1800 cSt at 40° C. available from DOW Chemical Corporation under the trade name UCON® 75-H-9500. 
     The fumed silica gel former may be a hydrophilic type fumed silica having a BET surface area of from 50 to 400 m 2 /g, such as 90 to 400 m 2 /g, preferably from 200 to 300 m 2 /g; or the fumed silica gel former may be a hydrophobized type fumed silica having has a BET surface area of from 50 to 300 m 2 /g, preferably from 250 to 350 m 2 /g; or mixtures of such hydrophilic and hydrophobized type fumed silica gel formers. 
     One of a gel former useful in the practice of the invention is a hydrophilic fumed silica having a BET surface of 300 m 2 /g available from EVONIK under the trade name Aerosil A300. 
     The gelling effect of the gel formers on the oils is accomplished by the formation of a network of the molecules of the gel former through hydrogen bonding via hydroxy groups or via van der Waals attraction between segments molecules of the gel former. The number and the strength of these bonds determines the gel strength, and the ability of the gel to support a load (critical yield stress). 
     The compositions of the invention are typically made by mixing together the ingredients, if necessary under slight heating to below 40° C. 
     Using the Compositions of the Invention 
     It is possible to balance tyres with the gels according to the present invention by simply applying a suitable amount of the gel to the inner lining of a tyre as is. However, since all parts of the gel must be interconnected, the whole surface of the “flat” part of the inner lining must be then gel covered, and gel in excess of the amount theoretically needed for balancing will have to be applied. 
     It has been determined that restriction of the gel to the shoulder areas of the inner lining of a tyre greatly increases the balancing efficiency of the gel and also drastically reduces the amount of gel needed for balancing. 
     In one embodiment, this restriction of movement can be achieved by attaching a foam strip to the centre part of the inner lining (see  FIG. 1 ). In a standard 245/45R18 tyre this foam strip is typically 100 mm wide and 5 mm thick. In another embodiment, the restriction can be achieved by attaching two separate strips of foam (10 mm high and 100 mm wide) near the shoulders of the tyre (see  FIG. 1 ). The amount of gel applied as an even layer on each shoulder is 2×80 g (in contrast the more than 300 g of gel usually required to balance an unmodified tyre). Furthermore the shoulder area in contact with the composition may be treated with a friction reducing nano-coating. 
     All tests with gel balancing in the following were carried out on tyres with the inner lining modified in this manner. 
     EXAMPLES 
     Using base liquids and gel formers as described above, a series of exemplary balancing gels have been prepared, and their compositions are shown in Table 1 and Table 2 
     
       
         
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 GEL FORMULATIONS. BALANCING RESULTS IN ROAD AND BENCH TESTS. CENTRIFUGAL STABILITY 
               
             
          
           
               
                 FORMULA # 
                 AEROSIL 
                 UCON 
                 UCON 
                 BALANCING 
                 BALANCING 
                 CENTRIFUGAL 
               
               
                 COMPONENT 
                 A300 
                 75-HB-9500 
                 50-HB-5100 
                 Bench test*  
                 Road test** 
                 STABILITY*** 
               
               
                   
               
             
          
           
               
                 1 
                 6 
                 0.5 
                 93.5 
                 − 
                 6 
                 + 
               
               
                 2 
                 7 
                 0.5 
                 92.5 
                 − 
                 5 
                 + 
               
               
                 3 
                 8 
                 0.5 
                 91.5 
                 − 
                 5 
                 + 
               
               
                 4 
                 6 
                 1 
                 93 
                 + 
                 7 
                 + 
               
               
                 5 
                 6.2 
                 1 
                 92.8 
                 + 
                 9 
                 + 
               
               
                 6 
                 6.3 
                 1 
                 92.7 
                 + 
                 10 
                 + 
               
               
                 7 
                 6.4 
                 1 
                 92.6 
                 + 
                 9 
                 + 
               
               
                 8 
                 6.5 
                 1 
                 92.5 
                 + 
                 8 
                 + 
               
               
                 9 
                 6.7 
                 1 
                 92.3 
                 + 
                 7 
                 + 
               
               
                 10 
                 7 
                 1 
                 92 
                 − 
                 5 
                 + 
               
               
                 11 
                 8 
                 1 
                 91 
                 − 
                 3 
                 + 
               
               
                 12 
                 5 
                 2 
                 93 
                 + 
                 7 
                 + 
               
               
                 13 
                 5.5 
                 2 
                 92.5 
                 + 
                 9 
                 + 
               
               
                 14 
                 5.8 
                 2 
                 92.2 
                 + 
                 10 
                 + 
               
               
                 15 
                 6 
                 2 
                 92 
                 + 
                 9 
                 + 
               
               
                 16 
                 6.5 
                 2 
                 91.5 
                 + 
                 7 
                 + 
               
               
                 17 
                 7 
                 2 
                 91 
                 − 
                 5 
                 + 
               
               
                 18 
                 8 
                 2 
                 90 
                 − 
                 4 
                 + 
               
               
                 19 
                 5 
                 3 
                 92 
                 + 
                 8 
                 − 
               
               
                 20 
                 5.5 
                 3 
                 91.5 
                 + 
                 10 
                 − 
               
               
                 21 
                 5.8 
                 3 
                 91.2 
                 + 
                 8 
                 − 
               
               
                 22 
                 6 
                 3 
                 91 
                 + 
                 7 
                 − 
               
               
                 23 
                 6.5 
                 3 
                 90.5 
                 − 
                 5 
                 − 
               
               
                 24 
                 7 
                 3 
                 90 
                 − 
                 4 
                 − 
               
               
                 25 
                 5 
                 5 
                 90 
                 + 
                 10 
                 − 
               
               
                 26 
                 5.5 
                 5 
                 89.5 
                 + 
                 8 
                 − 
               
               
                 27 
                 5.8 
                 5 
                 89.2 
                 − 
                 7 
                 − 
               
               
                 28 
                 6 
                 5 
                 89 
                 − 
                 6 
                 − 
               
               
                 29 
                 6.5 
                 5 
                 88.5 
                 − 
                 5 
                 − 
               
               
                 30 
                 7 
                 5 
                 88 
                 − 
                 4 
                 − 
               
               
                 31 
                 5 
                 10 
                 85 
                 + 
                 10 
                 − 
               
               
                 32 
                 5.5 
                 10 
                 84.5 
                 + 
                 8 
                 − 
               
               
                 33 
                 5.8 
                 10 
                 84.2 
                 + 
                 7 
                 − 
               
               
                 34 
                 6 
                 10 
                 84 
                 − 
                 6 
                 − 
               
               
                 35 
                 6.5 
                 10 
                 83.5 
                 − 
                 5 
                 − 
               
               
                 36 
                 7 
                 10 
                 83 
                 − 
                 4 
                 − 
               
               
                   
               
               
                 *“+” comparable or better than metal, “−” inferior to metal weight balancing. 
               
               
                 **7-10 Acceptable, &lt;7 Not acceptable 
               
               
                 ***Acceptable = +; Not acceptable = −  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 GEL FORMULATIONS. BALANCING RESULTS IN ROAD AND BENCH TESTS. CENTRIFUGAL STABILITY 
               
             
          
           
               
                 FORMULA # 
                 AEROSIL 
                 UCON 
                 SYNALOX 
                 SYNALOX 
                 BALANCING 
                 BALANCING 
                 CENTRIFUGAL 
               
               
                 COMPONENT 
                 A300 
                 75HB-9500 
                 D40-700 
                 D50-700 
                 Bench test  
                 Road test 
                 STABILITY 
               
               
                   
               
             
          
           
               
                 1 
                 7 
                 0 
                 93 
                   
                 − 
                 4 
                 + 
               
               
                 2 
                 7 
                 0.5 
                 92.5 
                   
                 − 
                 5 
                 + 
               
               
                 3 
                 7 
                 1 
                 92 
                   
                 + 
                 6 
                 + 
               
               
                 4 
                 7 
                 3 
                 90 
                   
                 + 
                 8 
                 − 
               
               
                 5 
                 7 
                 5 
                 88 
                   
                 + 
                 7 
                 − 
               
               
                 6 
                 7 
                 10 
                 83 
                   
                 + 
                 7 
                 − 
               
               
                 7 
                 6 
                 0 
                   
                 94 
                 − 
                 4 
                 + 
               
               
                 8 
                 6 
                 0.5 
                   
                 93.5 
                 − 
                 4 
                 + 
               
               
                 9 
                 6 
                 1 
                   
                 93 
                 + 
                 7 
                 + 
               
               
                 10  
                 6.3 
                 0 
                   
                 93.7 
                 − 
                 4 
                 + 
               
               
                 11  
                 6.3 
                 0.1 
                   
                 93.6 
                 − 
                 4 
                 + 
               
               
                 12  
                 6.3 
                 0.5 
                   
                 93.2 
                 + 
                 5 
                 + 
               
               
                 13  
                 6.3 
                 1 
                   
                 92.7 
                 + 
                 7 
                 + 
               
               
                 14  
                 6.5 
                 0 
                   
                 93.5 
                 − 
                 4 
                 + 
               
               
                 15  
                 6.5 
                 0.5 
                   
                 93 
                 + 
                 6 
                 + 
               
               
                 16  
                 6.5 
                 1 
                   
                 92.5 
                 + 
                 9 
                 + 
               
               
                 17  
                 6.5 
                 2 
                   
                 91.5 
                 + 
                 8 
                 − 
               
               
                 18  
                 7 
                 0 
                   
                 93 
                 − 
                 4 
                 + 
               
               
                 19  
                 7 
                 0.5 
                   
                 92.5 
                 + 
                 6 
                 + 
               
               
                 20  
                 7 
                 1 
                   
                 92 
                 + 
                 6 
                 + 
               
               
                 21* 
                 7 
                 93 
                   
                   
                 − 
                 4 
                 − 
               
               
                 22* 
                 6 
                 94 
                   
                   
                 + 
                 4 
                 − 
               
               
                 23* 
                 5 
                 95 
                   
                   
                 + 
                 4 
                 − 
               
               
                 24* 
                 4 
                 96 
                   
                   
                 + 
                 7 
                 − 
               
               
                 *Solidifies 
                   
                   
                   
                   
                 “+” comparable or 
                 7-10 
                 Acceptable = + 
               
               
                 below 
                   
                   
                   
                   
                 better than 
                 Acceptable 
                 Not acceptable = − 
               
               
                 −10° C. 
                   
                   
                   
                   
                 metal weights 
                 &lt;7 Not 
                   
               
               
                   
                   
                   
                   
                   
                 “−” inferior to 
                 acceptable 
                   
               
               
                   
                   
                   
                   
                   
                 metal weights 
               
               
                   
               
             
          
         
       
     
     The resulting compositions were evaluated by rheometry with a computerized Bohlin VOR Rheometer System (from Bohlin Rheology, Lund, Sweden) and in bench tests as described below and in field test using passenger cars. 
     The car tyres were modified as described above and supplied with 2×85 g of the balancing gels by applying the gel to the shoulder areas of the inner perimeter of the tyre before mounting. 
     The efficacy of a composition was evaluated in the following manner: 
     Bench test for measuring balancing efficiency of gels. 
     No matter what tyre balancing technique is used its efficiency ultimately is defined in terms of driving comfort, i.e. no vibrations or at least a tolerable level of vibrations in actual road tests. This obviously is a cumbersome method for quality control and sophisticated equipment has been developed for metal weight balancing of wheel-tyre assemblies that are all based on detection and compensation (with metal weights attached to the rim) of weight imbalance. The measurements are always carried out on wheel-tyre assemblies without load, and experience has shown that this procedure leads to the best possible reduction of weight imbalance induced vibrations in vehicle operation using rim attached metal weights. 
     This type of bench test equipment cannot be used to measure the efficiency of the gel balancing method or to determine the amount of gel to be used. The balancing gel inside a tyre is distributed in response to the actual vibrations in the wheel-tyre assembly. These vibrations are very different when operating under load (actual driving) than when operating without a load in testing equipment. Furthermore the distribution of balancing gel in an optimally gel-balanced tyre usually does not lead to a wheel-tyre assembly with zero weight imbalance, and this residual weight imbalance cannot be related to the efficiency of the gel balancing. 
     Consequently, a bench test for measuring the balancing efficiency of gels has been devised. In this bench test the actual driving conditions are simulated as closely as possible. The set-up is basically a modification of the drum test equipment used in the tyre industry, i.e. a variable speed rotating drum with a diameter around 300 mm, a moveable arm with an axle for mounting the wheel-tyre assembly and a device that allows varying the load of the tyre on the drum through a spring-dampening device. Acceleration sensors are mounted on the axle that allow measurement of vibration levels. In the set-up, only vertical accelerations are measured. The vertical accelerations vary constantly between two extremes (“up” and “down” direction) and the average difference (in g equivalents) between these two extremes is used as a measure of the vibration level (MVA value). 
     In order to establish an acceptance standard for gel balance the vertical accelerations were measured as described above at various speeds (between 80 and 180 kilometer per hour) and loads (from 300 to 1000 N) for a large number of tyres balanced to zero with metal weights. The MVA values obtained in this manner are the bench marks used for determining the performance of the balancing gel and used firstly to optimize gel performance and secondly to optimize the amount of balancing gel used. 
     In Table 1 and 2 are shown the results of such experiments 
     The stability of the balancing gels under high g-stress was measured with an ultracentrifuge. A sample of the gel was subjected to ˜300 000×g for 12 hours at 25° C. The oil that separated at the top of the sample in the centrifuge tube was decanted and weighed and the % oil loss (separation due to synresis) determined in this manner was used as a measure of the gel&#39;s stability under high g-stress. 
     From experience we know that an oil loss of less than 20% signifies that the gel will remain stable under the g-stress experienced in a tyre (&lt;1500 g) From the results in Table 1 and 2 as well as from experiments with a large number of other gels prepared from various PAG&#39;s and combinations of PAG&#39;s using a number of different fumed silicas as gel formers we can draw the following general conclusions: 
     To satisfy the separation stability requirements 
     PAG MW&gt;2000 
     PAG&#39;s or combinations of PAG gels for which 3/1&gt;PAG EO:PO ratio&gt;2/3 
     Hydrophobic fumed silica with a BET surface area&gt;150 
     Gels that satisfy the separation stability requirements above all have a certain balancing effect. The optimum balancing effect was achieved using a hydrophobic fumed silica with a BET surface of 300 in a PAG with a EO:PO ratio of 1:1 with a 0.5-1% additive of a PAG with a EO:PO ratio of 3:1.