Abstract:
A urethane bumper spring is produced from a urethane raw material containing a polyester-based polyol as a polyol component and diphenylmethane diisocyanate as an isocyanate component, and includes: a skin layer; and a core portion, in which the skin layer has a density (Da) and a foamed cell diameter (Ra), and the core portion has a density (Db) and a foamed cell diameter (Rb). The density (Da), the foamed cell diameter (Ra), the density (Db), and the foamed cell diameter (Rb) satisfy relationships shown in the following expressions: 1.0≦Da/Db&lt;1.34 (1) and 0.53&lt;Ra/Rb≦1.0 (2). Accordingly, the urethane bumper spring can achieve a cost reduction while having durability and settling resistance under high load and high deformation.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation of International Application No. PCT/JP2015/55327, filed on Feb. 25, 2015, which claims priority to Japanese Patent Application No. 2014-063589, filed on Mar. 26, 2014, the entire contents of each of which are hereby incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a urethane bumper spring configured to be mounted onto a piston rod of a shock absorber in a suspension mechanism of a vehicle so as to elastically restrain an operation stroke of the shock absorber, and to a method of producing the same. 
       BACKGROUND ART 
       [0003]    For example, as illustrated in  FIG. 1 , a substantially tubular (bellows-shaped) bumper spring  2  formed of an elastic material, e.g., a rubber or a foamed polyurethane resin is externally fitted onto a piston rod  31  of a shock absorber  30  serving as a component of a suspension of a vehicle, and the bumper spring  2  is arranged between a cylinder (absorber plate)  32  of the shock absorber  30 , and a fitting portion thereof on a vehicle body side (upper support  33 ) (see Patent Literature 1). 
         [0004]    As a typical example of the bumper spring for a vehicle, there is given a urethane bumper spring using a foamed polyurethane resin (hereinafter abbreviated as “urethane”). In addition, as a method of producing the urethane bumper spring, mold-forming involving using a forming mold (die) is generally used (see Patent Literature 2). 
         [0005]    The mold-forming of the urethane is a method of obtaining a urethane bumper spring as a product involving: injecting (casting) a urethane raw material into a forming cavity of a forming mold (die) formed of a master mold and a core, followed by heating, to thereby cure the urethane raw material through a reaction (foaming) in the forming mold; and then removing the resultant from the forming mold, followed by extraction of the forming mold core inside a tubular urethane molded body (demolding). As a foaming/curing method for the urethane, there is known a method involving two-stage heating steps, i.e., primary vulcanization and secondary vulcanization. In that case, the forming mold core inside the urethane molded body subjected to the primary vulcanization (semi-cured article) is extracted (demolding is performed) before the urethane molded body is further heated to be subjected to secondary vulcanization, and thus the urethane bumper spring is obtained as a completely cured article (product) (see Patent Literature 3). 
       RELATED ART DOCUMENTS 
     Patent Documents 
       [0006]    PTL 1: JP-B2-3758343 
         [0007]    PTL 2: JP-A-2004-293697 
         [0008]    PTL 3: JP-A-2006-56132 
       SUMMARY OF INVENTION 
       [0009]    A urethane bumper spring is required to withstand high load and high deformation, and hence a urethane raw material containing 1,5-naphthalene diisocyanate (NDI) as an isocyanate component is generally used for the urethane bumper spring. However, there is a problem in that such urethane bumper spring is difficult to apply to a general type of vehicle owing to high material cost of NDI. Accordingly, there is a demand for a bumper spring capable of achieving a cost reduction while having durability and settling resistance under high load and high deformation. 
         [0010]    The disclosed urethane bumper spring is capable of achieving a cost reduction while having durability and settling resistance under high load and high deformation, and a method of producing the same is disclosed. 
         [0000]    According to a first aspect, there is provided an urethane bumper spring having a hollow cylindrical shape, which is produced by mold forming and which has a skin layer in an outer layer thereof, the urethane bumper spring being configured to be fitted onto a piston rod of a shock absorber of a vehicle so as to elastically restrain an operation stroke of the shock absorber, the urethane bumper spring being produced from a urethane raw material containing a polyester-based polyol as a polyol component and diphenylmethane diisocyanate as an isocyanate component, the urethane bumper spring including: a skin layer; and a core portion, in which the skin layer has a density (Da) and a foamed cell diameter (Ra), and the core portion has a density (Db) and a foamed cell diameter (Rb), the density (Da), the foamed cell diameter (Ra), the density (Db), and the foamed cell diameter (Rb) satisfying relationships shown in the following expressions (1) and (2). 
         [0000]      1.0≦ Da/Db&lt; 1.34  (1)
 
         [0000]      0.53&lt; Ra/Rb≦ 1.0  (2)
 
         [0011]    Further, according to a second aspect, there is provided a method of producing the urethane bumper spring of the first aspect, the method including: a step of injecting a urethane raw material containing a polyester-based polyol as a polyol component and diphenylmethane diisocyanate as an isocyanate component into a cavity of a forming mold, followed by heating at 70° C. or more, to thereby subject the urethane raw material to primary vulcanization in the forming mold; a demolding step of removing a urethane molded body after the primary vulcanization from the forming mold; and a step of heating the urethane molded body after the demolding to subject the urethane molded body to secondary vulcanization. 
         [0012]    That is, the inventors have made extensive investigations in order to achieve the above-mentioned object. During the course of the investigations, the inventors have considered the use of a urethane raw material containing low-cost diphenylmethane diisocyanate (MDI) as an isocyanate component. As described above, however, there are problems in durability and settling resistance. The inventors have made further investigations in order to ascertain the cause of the problems, and as a result, have obtained the following finding: owing to notable differences in density and foamed cell diameter between the skin layer and the core portion of the urethane bumper spring, an internal crack (fracture) due to strain (stress concentration) is liable to occur in the bumper spring using the urethane raw material containing MDI as an isocyanate component, with the result that such bumper spring is poor in durability and settling resistance. In the case of the urethane bumper spring produced by mold forming, as described above, the skin layer is formed in its outer layer (portion brought into contact with a forming mold). When the skin layer poses the problems as described above, a measure involving cutting the skin layer after demolding is conceivable, but such measure still has, for example, the following problems: dimensional accuracy is not obtained; and a production process is complicated. In view of the foregoing, the inventors have made investigations on causing the skin layer and the core portion to have more similar densities and more similar foamed cell diameters, and as a result, have ascertained that, through the optimization of the polyol component, the optimization of the production method, and the like, the skin layer and the core portion can be caused to have more similar densities and more similar foamed cell diameters even in the case of the urethane raw material containing MDI as an isocyanate component. That is, the inventors have found that the desired object can be achieved when the densities and the foamed cell diameters of the skin layer and the core portion satisfy the specific relationships. Thus, the inventors have reached the embodiment of the present invention. 
         [0013]    The urethane bumper spring is a hollow cylindrical one produced by mold forming, and is produced from the urethane raw material containing the polyester-based polyol as a polyol component and diphenylmethane diisocyanate as an isocyanate component. In the urethane bumper spring, the density and the foamed cell diameter of the skin layer, and the density and the foamed cell diameter of the core portion satisfy the specific relationships. Accordingly, the urethane bumper spring is excellent in durability and settling resistance under high load and high deformation, and besides, can achieve a cost reduction. 
         [0014]    Further, the method of producing the urethane bumper spring includes: a step of injecting the urethane raw material containing a polyester-based polyol as a polyol component and diphenylmethane diisocyanate as an isocyanate component into the cavity of the forming mold, followed by heating at 70° C. or more, to thereby subject the urethane raw material to primary vulcanization in the forming mold; a demolding step of removing the urethane molded body after the primary vulcanization from the forming mold; and a step of heating the urethane molded body after the demolding to subject the urethane molded body to secondary vulcanization. Through the use of such production method, the urethane bumper spring as described above can be satisfactorily produced. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is an explanatory view illustrating a urethane bumper spring according to one embodiment of the present invention. 
           [0016]      FIG. 2A  and  FIG. 2B  are electron micrographs showing a cross-section of a skin layer and a cross-section of a core portion, respectively, in a urethane bumper spring of the embodiment of the present invention. 
           [0017]      FIG. 3A  and  FIG. 3B  are electron micrographs showing a cross-section of a skin layer and a cross-section of a core portion, respectively, in a related-art urethane bumper spring. 
           [0018]      FIG. 4  is an explanatory view illustrating a measurement method for a durability test of a urethane bumper spring in Examples. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0019]    Now, embodiments of the present invention are described in detail. 
         [0020]    A urethane bumper spring of the embodiments of the present invention is one to be fitted onto a piston rod of a shock absorber in a suspension system of an automobile so as to serve as a stopper or the like configured to elastically restrain an operation stroke of the shock absorber. The external appearance of the urethane bumper spring is, for example, a hollow cylindrical shape as with a related-art urethane bumper spring as illustrated in  FIG. 1 . 
         [0021]    The urethane bumper spring is produced by mold forming, and hence has a skin layer in its outer layer (portion brought into contact with a forming mold) as with a related-art urethane bumper spring produced by mold forming. However, the urethane bumper spring is produced from a urethane raw material containing a polyester-based polyol as a polyol component and diphenylmethane diisocyanate (MDI) as an isocyanate component, and includes: a skin layer; and a core portion, in which the skin layer has a density (Da) and a foamed cell diameter (Ra), and the core portion has a density (Db) and a foamed cell diameter (Rb), the density (Da), the foamed cell diameter (Ra), the density (Db), and the foamed cell diameter (Rb) satisfying relationships shown in the following expressions (1) and (2). The “skin layer” refers to a portion within 2 mm from a molded body surface in a thickness direction, and the “core portion” refers to a portion deeper than the skin layer. 
         [0000]      1.0≦ Da/Db&lt; 1.34  (1)
 
         [0000]      0.53&lt; Ra/Rb≦ 1.0  (2)
 
         [0022]      FIG. 2A  and  FIG. 2B  are electron micrographs (magnification of 50) showing a cross-section of a skin layer and a cross-section of a core portion, respectively, in the urethane bumper spring.  FIG. 3A  and  FIG. 3B  are electron micrographs (magnification of 50 times) showing a cross-section of a skin layer and a cross-section of a core portion, respectively, in the related-art urethane bumper spring. It is found from the cross-sectional photographs that the urethane bumper spring has less difference in foamed state between the skin layer and the core portion than in the related art. The skin layer and the core portion in the urethane bumper spring of  FIG. 2  satisfy the relationships shown in the expressions (1) and (2), whereas the skin layer and the core portion in the related-art urethane bumper spring of  FIG. 3  do not satisfy the relationships shown in the expressions (1) and (2). 
         [0023]    In the urethane bumper spring, the density (Da) of the skin layer and the density (Db) of the core portion need to satisfy the relationship shown in the expression (1). The density (Da) of the skin layer falls within the range of preferably from 0.4 g/cm 3  to 0.7 g/cm 3 , more preferably from 0.5 g/cm 3  to 0.6 g/cm 3 . In addition, the density (Db) of the core portion falls within the range of preferably from 0.4 g/cm 3  to 0.7 g/cm 3 , more preferably from 0.5 g/cm 3  to 0.6 g/cm 3 . 
         [0024]    In addition, in the urethane bumper spring, the foamed cell diameter (Ra) of the skin layer and the foamed cell diameter (Rb) of the core portion need to satisfy the relationship shown in the expression (2). The foamed cell diameter (Ra) of the skin layer falls within the range of preferably from 25 μm to 400 μm, more preferably from 50 μm to 300 μm. In addition, the foamed cell diameter (Rb) of the core portion falls within the range of preferably from 25 μm to 400 μm, more preferably from 50 μm to 300 μm. 
         [0025]    The density of each of the skin layer and the core portion is measured for a sample of the corresponding site using, for example, an automatic densimeter DSG-1 manufactured by Toyo Seiki Seisaku-sho, Ltd. In addition, the foamed cell diameter of each of the skin layer and the core portion is obtained by, for example, measuring, on the basis of cross-sectional photographs taken with an electron microscope as described above, the 10-point average of the diameters (cell diameters) of the foamed cells (hollow cells) thereof. 
         [0026]    In the urethane bumper spring, as described above, despite the use of the urethane raw material containing diphenylmethane diisocyanate (MDI) as an isocyanate component, the density and the foamed cell diameter of the skin layer and the density and the foamed cell diameter of the core portion satisfy the above-mentioned relationships mainly by virtue of the optimization of the polyol component and the optimization of the production method. 
         [0027]    That is, the polyester-based polyol is used as the polyol component in the urethane raw material. The polyester-based polyol is not particularly limited, but it is preferred that a polyester-based polyol formed of a condensation polymer of ethylene glycol, butanediol, and adipic acid be used as the polyester-based polyol. This is because, when the urethane raw material containing the specific polyester-based polyol and MDI is used and, as described later, primary vulcanization is performed with the temperature of a forming mold set to high temperature, the urethane bumper spring including the skin layer and the core portion as described above can be satisfactorily produced. 
         [0028]    The polyester-based polyol formed of a condensation polymer of ethylene glycol, butanediol, and adipic acid has low freezing point. Accordingly, for example, when a prepolymer method is adopted, work involving melting the polyester-based polyol in advance before the preparation of a prepolymer is not required, and hence the preparation of the prepolymer is facilitated. In addition, by virtue of the low freezing point, a one-shot method, which includes a smaller number of steps than the prepolymer method, can also be adopted. As just described, the use of the specific polyester-based polyol provides advantages in terms of production method as well. 
         [0029]    In addition, the polyester-based polyol preferably has a number-average molecular weight (Mn) of from 500 to 4,000, and the number-average molecular weight (Mn) more preferably falls within the range of from 1,000 to 3,000. This is because, when the polyester-based polyol having such number-average molecular weight is used, the bumper spring having the characteristic structure as described above can be satisfactorily produced. The number-average molecular weight (Mn) may be determined by gel permeation chromatography measurement or the like. 
         [0030]    Examples of the diphenylmethane diisocyanate (MDI) to be used with the polyester-based polyol include 2,2′-MDI, 2,4′-MDI, 4,4′-MDI, and polymeric MDI. One kind of those diphenylmethane diisocyanates is used alone, or two or more kinds thereof are used in combination. 
         [0031]    In the urethane bumper spring, as described above, the urethane raw material to be used contains the polyester-based polyol as a polyol component and diphenylmethane diisocyanate (MDI) as an isocyanate component. In addition, with regard to the blending ratio of the isocyanate component with respect to the polyol component, it is preferred that the components be blended so that the NCO index of the urethane raw material [equivalent ratio of an NCO group in the isocyanate to a hydroxy group in the polyol (NCO group/OH group)] may fall within the range of from 0.9 to 1.3 from the viewpoint of satisfactorily producing the bumper spring having the characteristic structure as described above. From a similar viewpoint, the NCO index more preferably falls within the range of from 1.0 to 1.2. 
         [0032]    In addition, the urethane raw material contains the polyol component, the isocyanate component, and as necessary, a foaming agent, such as water, a chain extender, a catalyst, a foam stabilizer, a hydrolysis inhibitor, a flame retardant, a viscosity reducer, a stabilizer, a filler, a cross-linking agent, a colorant, or the like. 
         [0033]    Herein, a method of producing the urethane bumper spring includes, for example: 
         [0000]    1) a die preparing step of preparing a forming mold (die) formed of a master mold and a core;
 
2) a primary vulcanization step of injecting (casting) the urethane raw material into the forming cavity of the die, followed by heating, to thereby foam and cure (semi-cure) the urethane raw material to obtain a molded body (semi-processed product) for a bumper spring;
 
3) a demolding step of removing the bumper spring molded body after the primary vulcanization from the master mold, followed by the extraction of the core from the inner periphery of the molded body; and
 
4) a secondary vulcanization step of heating the bumper spring molded body after the demolding to complete the curing reaction of the urethane, to thereby obtain a urethane bumper spring as a product.
 
         [0034]    The urethane bumper spring is specifically produced as described below in accordance with the steps 1) to 4). 
         [0035]    [Die Preparing Step] 
         [0036]    First, a die capable of forming a cavity having the same shape as the bumper spring (see  FIG. 1 ) is prepared. In addition, as necessary, the surface (outer peripheral surface) of its core is coated with a release agent by means of a spray or any other method, followed by drying, to form a uniform coating film formed of the release agent. Then, the master mold and the core are combined to form a predetermined forming cavity, and then the die is heated to a predetermined temperature (70° C. or more). 
         [0037]    [Primary Vulcanization Step] 
         [0038]    Next, the urethane raw material containing the polyester-based polyol as a polyol component and diphenylmethane diisocyanate as an isocyanate component is injected into the cavity of the forming mold, and is heated at 70° C. or more (preferably from 70° C. to 130° C.) for from 2 minutes to 60 minutes to be foamed and cured into polyurethane foam. Thus, a bumper spring molded body subjected to primary vulcanization (semi-processed product) is produced. When the heating temperature is 150° C. or more, there is a risk in that a surface blister of the molded body may occur. In addition, the urethane raw material may be injected into the cavity of the forming mold after the preparation of a prepolymer in advance in accordance with the prepolymer method, or the polyol component and the isocyanate component may be mixed in the cavity of the forming mold in accordance with the one-shot method. 
         [0039]    [Demolding Step] 
         [0040]    Next, the bumper spring molded body after the completion of the primary vulcanization is removed from the master mold, and then the core is extracted from the inner periphery of the molded body. 
         [0041]    [Secondary Vulcanization Step] 
         [0042]    After that, the bumper spring molded body removed from the die as described above is heated using a heating furnace or the like in accordance with predetermined conditions to allow an unreacted polyol component and polyisocyanate component remaining in the molded body to sufficiently react. Thus, the curing reaction is completed to obtain a urethane bumper spring as a finished product. As heating conditions in the secondary vulcanization step, conditions of heating at a temperature of from about 100° C. to about 150° C. for from about 3 hours to about 20 hours are adopted. 
         [0043]    The thus obtained urethane bumper spring is suitable as a bumper spring to be mounted onto a piston rod of a shock absorber. 
       EXAMPLES 
       [0044]    Now, Examples are described together with Comparative Example. However, the embodiment of the present invention is not limited to these Examples, and modifications are possible without departing from the gist of the present disclosure. 
         [0045]    First, prior to Examples and Comparative Example, materials shown below were prepared. 
         [0046]    [Polyol Agent] 
         [0047]    A polyol agent which contains a polyester-based polyol as a polyol component thereof (Toral HT-2040, manufactured by DIC Corporation) 
         [0048]    [Chain Extender] 
         [0049]    Ethylene glycol 
         [0050]    [Catalyst] 
         [0051]    Toral SM-2, manufactured by DIC Corporation 
         [0052]    [Foaming Agent] 
         [0053]    Water 
         [0054]    [Isocyanate Agent] 
         [0055]    An isocyanate agent which contains MDI as an isocyanate component thereof (Toral F-2560, manufactured by DIC Corporation) 
       Example 1 
       [0056]    A liquid A formed of 100 parts by weight of a polyol agent, 11.8 parts by weight of a chain extender, 1.1 parts by weight of a catalyst, and 0.6 part by weight of a foaming agent was prepared. In addition, a liquid B formed of 169.3 parts by weight of an isocyanate agent was prepared. Next, a die (master mold and core) capable of forming a cavity having the same shape as a bumper spring  40  as illustrated in  FIG. 4  was prepared. Then, the master mold and the core were combined to form the predetermined forming cavity, and then the die formed of the master mold and the core was heated. Subsequently, the liquid A and the liquid B were stirred at 3,000 rpm for 5 seconds (liquid temperature: 40±1° C., NCO index: 1.0), and then the mixture was injected into the die and subjected to primary vulcanization for 10 minutes while the temperature of the die was kept at 70° C. Then, the urethane molded body after the primary vulcanization was demolded from the die, and further subjected to secondary vulcanization (110° C.×16 hours) in an oven. Thus, a urethane bumper spring of interest (height (A): 75 mm, inner diameter (B): 20 mm, outer diameter (C): 57 mm &lt;&lt;see  FIG. 4 &gt;&gt;) was produced. 
       Examples 2 to 4 and Comparative Example 1 
       [0057]    As shown in Table 1 below, the blending amounts of the respective components and the like were the same as those in Example 1, but the die temperature in the primary vulcanization was changed from that in Example 1. Thus, urethane bumper springs each having a shape as illustrated in  FIG. 4  (height (A): 75 mm, inner diameter (B): 20 mm, outer diameter (C): 57 mm &lt;&lt;see  FIG. 4 &gt;&gt;) was produced. 
         [0058]    For the urethane bumper springs of Examples and Comparative Example obtained as described above, various properties were measured and evaluated in accordance with criteria described below. The results of measurement and evaluation are also shown in Table 1 below. 
         [0059]    [Density] 
         [0060]    From each of a skin layer (within 2 mm from a molded body surface in a thickness direction) and a core portion (portion deeper than the skin layer) in a cross-section of a bumper spring, a sample was cut out, and the density Da (g/cm 3 ) of the skin layer and the density Db (g/cm 3 ) of the core portion were measured using an automatic densimeter DSG-1 manufactured by Toyo Seiki Seisaku-sho, Ltd. to determine Da/Db. The density measurement with the densimeter was performed a total of six times, i.e., at three sites in two samples for each of the skin layer and the core portion, and the average value of the six times of measurement was adopted as a measurement value. 
         [0061]    [Foamed Cell Diameter] 
         [0062]    Electron micrographs were taken of a skin layer (within 2 mm from a molded body surface in a thickness direction) and a core portion (portion deeper than the skin layer) in a cross-section of a bumper spring, and on the basis of the 10-point average of the diameters (cell diameters) of the (hollow) cells thereof, the foamed cell diameter Ra (μm) of the skin layer and the foamed cell diameter Rb (μm) of the core portion were measured to determine Ra/Rb. 
         [0063]    [Durability] 
         [0064]    As illustrated in  FIG. 4 , the bumper spring  40  was fitted onto a servo endurance testing machine including a fitting  43 , a piston rod  41 , and an abutment portion  42 , and was subjected to a repeated (200,000-time) compression test of from 0 N to 7,000 N at a frequency of 2 Hz. The bumper spring  40  has a height (A) of 75 mm, an inner diameter (B) of 20 mm, and an outer diameter (C) of 57 mm. In addition, the piston rod  41  has an outer diameter (D) of 12.5 mm, and the abutment portion  42  has an outer diameter (E) of 50 mm. In addition, the external appearance of the bumper spring was observed at an initial stage and after the 200,000-time compression test, and evaluation was performed by marking a case where a problem such as a crack was found after the compression test with Symbol “x”, and marking a case where a problem such as a crack was not found with Symbol “o”. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                   
                 Comparative 
               
               
                   
                 Example 
                 Example 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 1 
               
               
                   
                   
               
             
          
           
               
                 Polyol agent 
                 100 
                 100 
                 100 
                 100 
                 100 
               
               
                 Chain extender 
                 11.8 
                 11.8 
                 11.8 
                 11.8 
                 11.8 
               
               
                 Catalyst 
                 1.1 
                 1.1 
                 1.1 
                 1.1 
                 1.1 
               
               
                 Foaming agent 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
                 0.6 
               
               
                 Isocyanate agent 
                 169.3 
                 169.3 
                 169.3 
                 169.3 
                 169.3 
               
               
                 NCO index (NCO/OH) 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
                 1.0 
               
               
                 Die temperature (° C.) 
                 70 
                 90 
                 110 
                 130 
                 50 
               
             
          
           
               
                 Density 
                 Skin layer (Da) 
                 0.64 
                 0.63 
                 0.60 
                 0.56 
                 0.63 
               
               
                 (g/cm 3 ) 
                 Core portion (Db) 
                 0.49 
                 0.52 
                 0.52 
                 0.50 
                 0.47 
               
               
                   
                 Da/Db 
                 1.32 
                 1.22 
                 1.16 
                 1.13 
                 1.34 
               
               
                 Foamed cell diameter 
                 Skin layer (Ra) 
                 83 
                 115 
                 110 
                 114 
                 75 
               
               
                 (μm) 
                 Core portion (Rb) 
                 119 
                 124 
                 139 
                 131 
                 142 
               
               
                   
                 Ra/Rb 
                 0.70 
                 0.93 
                 0.79 
                 0.87 
                 0.53 
               
               
                 Durability 
                 At 200,000th time 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                   
               
             
          
         
       
     
         [0065]    As apparent from the results of Table 1, in the bumper spring of each of Examples, the problem such as a crack was not found even after the durability test, whereas in the bumper spring of Comparative Example 1, an internal crack occurred after the durability test owing to its excessively large value of Da/Db, and a crack was found in the external appearance. 
         [0066]    Although specific embodiments of the present invention have been described in Examples above, Examples are for illustrative purposes only and are not to be construed as limitative. It is intended that various modifications apparent to a person skilled in the art fall within the scope of the present invention. 
         [0067]    The urethane bumper spring is suitable as a bumper spring to be mounted onto a piston rod of a shock absorber.