Patent Publication Number: US-2018037721-A1

Title: Stabilized polyolefin resin material

Description:
FIELD OF INVENTION 
     The present invention relates to a stabilized resin material. In particular, the present invention relates to a resin material that is stabilized by a combination of a specific primary antioxidant(s) and a specific secondary antioxidant(s) and, especially, that is suppressed in discoloration. 
     BACKGROUND ART 
     When attacked by heat, light, or physical stress, resin materials, for example, polyolefins (RH), generate free radicals (R*), which react with oxygen to generate peroxy radicals (ROO*). The peroxy radicals can abstract hydrogens from other molecules (RH) to become hydroperoxides (ROOH) and, at the same time, further generate new free radicals (R*). The newly generated free radicals may trigger next reactions, that is, chain reactions occur, which leads to deterioration of the resin materials. 
     As the hydroperoxides (ROOH) are unstable, they decompose themselves further and generate new radicals (for example, RO*). The new radicals can abstract hydrogens from other molecules to increase radicals, which also results in deterioration of the resin materials. Such decomposition of molecules via hydroperoxides is referred to as autoxidation. 
     In order to prevent such decomposition of resin molecules, i.e., to prevent deterioration, antioxidants can be used. The antioxidants known in the art include ones that can mainly trap radicals to prevent autoxidation in its early stage (referred to as “primary antioxidants”), and also include ones that can suppress further generation of free radicals by decomposing the unstable hydroperoxides into stable products (referred to as “secondary antioxidants”). The primary antioxidants include, for example, phenol-based antioxidants and the like. The secondary antioxidants include, for example, phosphoric acid-based antioxidants, sulfur-based antioxidants, and the like. A combined use of the primary antioxidants and the secondary antioxidants is also known and exhibits a synergistic effect for preventing oxidation. 
     Furthermore, it is known that the phenol-based antioxidants, which are widely used as the primary antioxidants, become quinone compounds and then stilbene quinones, which may color or discolor the resins to which the antioxidants have been added. It is not desirable that the antioxidants added to prevent deterioration of the resin materials damage appearance thereof. Therefore, there remains a need to suppress discoloration of the resin materials when using the phenol-based antioxidants. 
     With regard to suppression of the discoloration, for example, JP 2002-527595 A (Patent Literature 1) discloses a group of antioxidant compounds that have structures similar to but not exactly the same as those of the present invention, in combination of a primary antioxidant(s) and a secondary antioxidant(s). Patent Literature 1 also discloses that the combination results in increased stability and whiteness of a polymer. However, the Patent Literature 1 has an object of solving disadvantages caused by addition of phosphite esters that are secondary antioxidants for preventing discoloration of the primary antioxidants, and presupposes presence of water. Further, JP 2005-082642 A (Patent Literature 2) discloses a group of compounds used in the present invention as antioxidants, but does not disclose that the compounds were used by specifically combining them nor, of course, confirm an effect of the combination. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] JP 2002-527595 A 
     [Patent Literature 2] JP 2005-082642 A 
     SUMMARY OF INVENTION 
     The present inventors have now found that, by using specific two kinds of secondary antioxidants in combination, discoloration (red to pink), which is considered to be caused by phenol-based antioxidants, can be effectively prevented for a long period of time. The present invention is based on such findings. 
     Accordingly, an object of the present invention is to provide a resin material containing a polyolefin, the resin material being effectively prevented from discoloration of the resin that occurs when a phenol-based antioxidant is used. 
     Further, an object of the present invention is to provide a combination of antioxidants that can be used in combination with a phenol-based antioxidant in a resin material containing a polyolefin as a main polymer component. 
     Furthermore, an object of the present invention is to provide an antioxidant for polyolefins and a green body. 
     The resin material according to the present invention comprises:
     (1) a polyolefin;   (2) a compound represented by the following formula (I):   

     
       
         
         
             
             
         
       
     
     wherein, R 1  and R 2  each independently represent a linear or branched hydrocarbon group having 1 to 20 carbon atoms;
     (3) a compound represented by the following formula (II):   

     
       
         
         
             
             
         
       
     
     wherein, R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  each independently represent a linear or branched hydrocarbon group having 1 to 20 carbon atoms; and
     (4) a phenol-based antioxidant.   

     Further, the antioxidant for polyolefins according to the present invention comprises a compound represented by the formula (I), a compound represented by the formula (II), and a phenol-based antioxidant. 
     Furthermore, the green body according to the present invention comprises the resin material according to the present invention. 
     According to the present invention, the discoloration (red to pink) of a resin material containing a polyolefin, which is considered to be caused by phenol-based antioxidants, can be effectively prevented for a long period of time. The present inventors have confirmed experimentally that the effect according to the present invention is obtained by a combination of at least two kinds of specific secondary antioxidants and is remarkable in comparison to that exhibited by other secondary antioxidants having similar structures. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     Resin Material 
     The resin material according to the present invention comprises at least (1) a polyolefin, (2) a compound of the formula (I), (3) a compound of the formula (II), and (4) a phenol-based antioxidant. According to the present invention, discoloration (red to pink) of the resin material, which is considered to be caused by the phenol-based antioxidant, can be effectively prevented for a long period of time, by a combination of the antioxidants (2), (3), and (4). The reason for this is not definite but is considered as follows. Various reasons may be presumed why the phenol-based antioxidant is changed to a stilbene quinone via a quinone-based compound. Among them, it is considered that the phenol-based antioxidant traps a radical and, as a result, changes its structure to finally become a stilbene quinone. In the present invention, it is considered that the compound of the formula (I) and the compound of the formula (II) interact synergistically to effectively prevent generation of a radical derived from an unstable hydroperoxide and efficiently suppress change in a color-developing structure of the phenol-based antioxidant. In particular, from comparison of Examples and Comparative Examples described later, it is considered desirable that one of ester structures of phosphorous acid has a structure of a phenyl substituted by alkyl groups at 2, 4, and 6 positions. The above is only a hypothesis, and the present invention should not be understood as being limited thereto. 
     The resin material according to the present invention contains a plurality of antioxidants in combination. In the present invention, however, the presence of a plurality of antioxidants has little effect on physical properties such as hardness and chemical resistance of the resin material. The present invention is also advantageous in this respect. 
     Meanwhile, the resin material according to the present invention allows presence of unavoidable water and does not essentially require addition of water. However, it is desirable to avoid the presence of water, because it generally causes defects in the molding process of the resin material resulting in poor appearance. 
     (1) Polyolefin 
     The resin material according to the present invention contains a polyolefin. In the present invention, polyolefin means a polymer of an olefinic compound, that is, a hydrocarbon having at least one double bond as a bond between carbon atoms, and may be either a homopolymer or a copolymer. The olefinic compound, that is, a monomer includes, for example, mono-1-olefins having 2 to 16 carbon atoms, preferably mono-1-olefins having 2 to 10 carbon atoms, and more preferably mono-1-olefins having 2 to 6 carbon atoms. Specific examples thereof include, though not limited thereto, ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and the like. Examples of polyolefins to which the present invention is favorably applied include polyethylene, polypropylene, and the like. 
     In one preferred embodiment of the present invention, as the polyolefin in the present invention, an olefin-based elastomer containing a polyolefin combined with a so-called elastomer component may be used. The elastomer component includes natural rubber, ethylene-propylene rubber, polybutadiene rubber, and the like. The mode of addition of the elastomer component may include blending by finely dispersing the elastomer component, addition accompanying chemical crosslinking. The present invention can be widely applied to heretofore known olefin-based elastomers as well as materials that will become known in the future and classified as olefin-based elastomers. 
     The resin material according to the present invention can be used as a polyolefin without any particular limitation. Especially, the material is preferably used in applications where discoloration impairs its commercial value. 
     (2) Compound of Formula (I) 
     In the present invention, the compound of the formula (I) is a secondary antioxidant. In the formula, R 1  and R 2  each independently represent a linear or branched hydrocarbon group, preferably alkyl group, having 1 to 20 carbon atoms, preferably 1 to 16 carbon atoms, and more preferably 1 to 8 carbon atoms. Examples of R 1  and R 2  include methyl, tert-butyl, and 1,1-dimethylpropyl. 
     Preferable examples of the compound of the formula (I) include tris (2,4-di-tert-butylphenyl) phosphite, tris (2-tert-butylphenyl) phosphite, and tris [2,4-di-(1,1-dimethylpropyl)phenyl] phosphite. 
     The addition amount of the compound of the formula (I) may be suitably determined by taking its effect into consideration. For example, the amount is about 0.01 to 1% by mass, and preferably about 0.05 to 0.5% by mass relative to the polymer component. 
     (3) Compound of Formula (II) 
     In the present invention, the compound of the formula (II) is also a secondary antioxidant. In the formula, R 3 , R 4 , R 5 , R 6 , R 7  , and R 8  each independently represent a linear or branched hydrocarbon group, preferably alkyl group, having 1 to 20 carbon atoms, preferably 1 to 16 carbon atoms, and more preferably 1 to 8 carbon atoms. Examples of R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  include methyl, tert-butyl, and 1,1-dimethylpropyl. 
     Preferable examples of the compound of the formula (II) include bis-(2,6-di-tert-butyl-4-methylphenyl)pentaerythritol diphosphite. 
     The addition amount of the compound of the formula (II) may be suitably determined by taking its effect into consideration. For example, the amount is about 0.01 to 1% by mass, and preferably about 0.05 to 0.5% by mass relative to the polymer component. 
     (4) Phenol-Based Antioxidant 
     The phenol-based antioxidant contained in the resin material according to the present invention prevents deterioration of polyolefins as a so-called primary antioxidant. In the present invention, phenol-based antioxidant means an antioxidant having a function of mainly trapping a radical. Phenol-based antioxidant is preferably one classified as a hindered phenol. The phenol-based antioxidant is selected from the group consisting of monophenols, bisphenols, thiobisphenols, polyphenols, hydroxybenzyl aromatic carboxylates, amides of β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid, esters of β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid with mono- or polyhydric alcohols, spiro compounds, and mixtures thereof. 
     The monophenols may include 2,6-di-tert-butyl-4-methylphenol, 2-tert-butyl-4-methoxyphenol, and 4-(hydroxymethyl)-2,6-di-tert-butyl phenol. 
     The bisphenols may include 2,2′-methylenebis(6-tert-butyl-4-methylphenol), 2,2′-methylenebis(6-tert-butyl-4-ethylphenol), 2,2-methylenebis[4-methyl-6-(α-methylcyclohexyl)phenol], 1,1-bis(5-tert-butyl-4-hydroxy-2-methylphenyl)butane, 2,2-bis(5-tert-butyl-4-hydroxy-2-methylphenyl)butane, 2,2-bis-(3,5-di-tert-butyl-4-hydroxyphenyl)propane, 1,1,3-tris(5-tert-butyl-4-hydroxy-2-methylphenyl)butane, 2,2-bis(5-tert-butyl-4-hydroxy-2-methylphenyl)-4-n-dodecylmercaptobutane, 1,1,5,5-tetra(5-tert-butyl-4-hydroxy-2-methyl-phenyl)pentane, ethylene glycol-bis[3,3-bis (3′-tert-butyl-4′-hydroxyphenyl) butyrate], 1,1-bis(3,5-dimethyl-2-hydroxy phenyl)-3-(n-dodecylthio)butane, and 4,4′-thiobis(6-tert-butyl-3-methylphenol). 
     The thiobisphenols may include 4,4′-thiobis(6-tert-butyl-m-cresol), 1,1′-thiobis(2-naphthol), and 2,2′-thiobis(4-methyl-6-tert-butyl phenol). 
     The polyphenols may include tetrakis(methylene-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene, 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl) isocyanurate, and tetrakis[methylene-(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)]methane. 
     The hydroxybenzyl aromatic carboxylates may include 1,3,5-tri(3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene, 2,2-bis(3,5-di-tert-butyl-4-hydroxybenzyl)malonic acid dioctadecyl ester, 1,3,5-tris(3,5-di-test-butyl-4-hydroxybenzyl) isocyanurate, and 3,5-di-tert-butyl-4-hydroxybenzylphosphonic acid diethyl ester. 
     The amides of β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionic acid may include 1,3,5-tris(3,5-di-tert-butyl-4-hydroxyphenyl-propionyl-hexahydro-s-triazine and N,N′-di(3,5-di-tert-butyl-4-hydroxyphenyl-propionyl)hexamethylenediamine. 
     The addition amount of the phenol-based antioxidant may be suitably determined by taking its effect into consideration. For example, the amount is about 0.01 to 1% by mass, and preferably about 0.05 to 0.5% by mass relative to the polymer component. 
     Other Components 
     To the resin material according to the present invention, various components, which are ordinarily or generally added arbitrarily to polyolefins, may be added. 
     Antioxidant and Green Body 
     As is clear from the above, according to one embodiment of the present invention, an antioxidant for polyolefins is provided, which comprises a compound represented by the formula (I), a compound represented by the formula (II), and a phenol-based antioxidant. Further, according to another embodiment of the present invention, a green body is provided, which comprises the resin material according to the present invention. The green body can be used in a desirable use without any change in its various physical properties even by addition of the antioxidant. Further, the green body is effectively prevented from discoloration. 
     EXAMPLES 
     The following Examples illustrate the present invention in more detail. However, it should be noted that the present invention is not limited to these Examples. In addition, unless otherwise noted, % means % by mass in the following. 
     1. Preparation of Samples 
     Resin materials having the following compositions were provided. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Example 
                 Example 
                 Comparative 
                 Comparative 
               
               
                   
                 1 
                 2 
                 Example 1 
                 Example 2 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Olefin-based 
                 99.7 
                 99.6 
                 99.8 
                 99.8 
               
               
                 elastomer 
                   
                   
                   
                   
               
               
                 Compound of 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 formula (I) 
                   
                   
                   
                   
               
               
                 Compound of 
                 0.1 
                 0.2 
                 — 
                 — 
               
               
                 formula (II) 
                   
                   
                   
                   
               
               
                 Compound of 
                 — 
                 — 
                 — 
                 0.2 
               
               
                 formula (III) 
                   
                   
                   
                   
               
               
                 Phenol-based 
                 0.1 
                 0.1 
                 0.1 
                 0.1 
               
               
                 antioxidant 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     In Table 1, specific materials are as follows:
     the olefin-based elastomer: an olefin-based thermoplastic elastomer with PP as the hard segment;   the compound of formula (I): a compound corresponding to the compound of formula (I) wherein R 1  and R 2  are tert-butyl, which is commercially available from ADEKA Corporation as “2112”;   the compound of formula (II): a compound corresponding to the compound of formula (II) wherein R 3 , R 4 , R 6 , and R 7  are tert-butyl, and R 5  and R 8  are methyl, which is commercially available from ADEKA Corporation as “PEP-360”; and   the compound of formula (III): a compound corresponding to the compound represented by the following structural formula (III):   

     
       
         
         
             
             
         
       
     
     which is commercially available from ADEKA Corporation as HP-10. 
     The resin materials having the above compositions were molded by injection molding method to obtain sample plates (10 cm×5 cm) and dumbbell test pieces. The injection molding conditions were such that the cylinder temperature was set at 190° C. and the mold temperature was set at 40° C. Further, toilet seat buffer pads, which are disposed on the toilet seat on the side which is contact with the toilet, were prepared in accordance with the above molding method. 
     2. Forced Circulation Type Heat Aging Test 
     The toilet seat buffer pads and the sample plates, obtained in the above 1, were placed in a forced circulation type heat aging tester (manufactured by ESPEC Corporation) and accelerated tests of resin deterioration by heat were carried out. The temperature was set at 70° C. The testing time was set at a maximum of 624 hours. 
     3. Measurement of Discoloration and the Result 
     The test pieces were taken out of the tester mentioned in the above 2 after the time shown in the following table had passed, and degrees of discoloration in comparison to initial test pieces were measured using a spectrophotometer CM-5 (manufactured by Konica Minolta, Inc.). Measured were a value, b value, and L value, and the degree of discoloration was expressed by Δa, a variation in a value (showing a color change in the red-green region), and the following ΔE as indices: 
       Δ E   2 =(Δ a   2   +Δb   2 )+Δ L   2  
 
     The results were as shown in the following tables 2 and 3. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Test piece: toilet seat buffer pad 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Example 
                 Example 
                 Comparative 
               
               
                 Time (hr) 
                   
                 1 
                 2 
                 Example 1 
               
               
                   
               
               
                 192 
                 ΔE 
                 0.44 
                 0.35 
                 1.97 
               
               
                   
                 Δa 
                 0.21 
                 0.12 
                 0.95 
               
               
                 312 
                 ΔE 
                 0.97 
                 0.63 
                 3.90 
               
               
                   
                 Δa 
                 0.46 
                 0.24 
                 2.07 
               
               
                 624 
                 ΔE 
                 2.13 
                 1.12 
                 4.17 
               
               
                   
                 Δa 
                 0.99 
                 0.48 
                 2.08 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Test piece: sample plate 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Example 
                 Example 
                 Comparative 
                 Comparative 
               
               
                 Time (hr) 
                   
                 1 
                 2 
                 Example 1 
                 Example 2 
               
               
                   
               
               
                 168 
                 ΔE 
                 1.03 
                 0.66 
                 4.50 
                 2.73 
               
               
                   
                 Δa 
                 0.33 
                 0.12 
                 2.73 
                 1.51 
               
               
                   
               
            
           
         
       
     
     It was confirmed that, in the resin materials according to the present invention, discoloration is remarkably suppressed in comparison to a case where only the compound of formula (I) is used and in comparison to a case where the compound of formula (I) is used in combination with an antioxidant having a different structure therefrom such as the compound of formula (III). 
     4. Evaluation of Physical Properties of Material 
     (a) Hardness 
     Hardness of the toilet seat buffer pad prepared in the above 1 was measured by using a durometer in accordance with JIS K 6253-3/ISO 7619-1. 
     (b) Tensile Strength 
     The tensile strength test was carried out by using the dumbbell test pieces prepared in the above 1. The test was performed in accordance with JIS K 6251/ISO 37 by stretching the test pieces using an autograph (manufactured by Shimadzu Corporation) at a rate of 500 mm/min, and the maximum stress was recorded. 
     (c) Tear Strength 
     Angle test pieces without notch were punched out of the sample plates prepared in the above 1, and the tear strength test was carried out using them. The test was performed in accordance with JIS K 6252/ISO 34-1 by stretching the test pieces using an autograph (manufactured by Shimadzu Corporation) at a rate of 500 mm/min, and the maximum stress was recorded. 
     (d) Compression Permanent Set 
     Using the sample plates prepared in the above 1, 25% compression permanent set was measured in accordance with JIS K 6262/ISO 815. 
     The results of the above tests were as shown in the table 4 described below. 
     5. Detergent Resistance 
     The toilet seat buffer pads prepared in the above 1 were immersed in the following detergents for 7 days, and the degree of discoloration was measured in accordance with the test method of the above 3:
     Acid-based detergent: Sanpol (Dainihon Jochugiku Co., Ltd.);   Alkali-based detergent: Phosphorus-free Forward Cleaner (CxS Corporation);   Chlorine-based detergent: Wide Hiter (Kao Corporation);   Neutral detergent: Toilet Magiclean (Kao Corporation).   

     The results of the above test were as shown in the following table 4. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                   
                   
                 Comparative 
               
               
                   
                   
                 Example 1 
                 Example 2 
                 Example 1 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Physical 
                 Hardness 
                 96 
                 96 
                 96 
               
               
                 properties 
                 Tensile 
                 8.2 
                 8.2 
                 8.0 
               
               
                   
                 strength [MPa] 
                   
                   
                   
               
               
                   
                 Tear strength 
                 105 
                 104 
                 105 
               
               
                   
                 [kN/m] 
                   
                   
                   
               
               
                   
                 Compression 
                 61 
                 61 
                 61 
               
               
                   
                 permanent set 
                   
                   
                   
               
               
                   
                 [%] 
                   
                   
                   
               
               
                 Detergent 
                 Acid-based 
                 0.31 
                 0.25 
                 0.37 
               
               
                 resistance 
                 detergent 
                   
                   
                   
               
               
                 [ΔE] 
                 Alkali-based 
                 0.15 
                 0.19 
                 0.15 
               
               
                   
                 detergent 
                   
                   
                   
               
               
                   
                 Chlorine-based 
                 0.13 
                 0.13 
                 0.24 
               
               
                   
                 detergent 
                   
                   
                   
               
               
                   
                 Neutral 
                 0.05 
                 0.11 
                 0.13 
               
               
                   
                 detergent 
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     From the table, it can be seen that addition of the antioxidant according to the present invention has little effect on the various physical properties of the resin.