Method for preventing or eliminating water-absorption whitening of a molded article containing an acrylonitrile polymer or copolymer
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for preventing or eliminating water-absorption whitening of a molded article containing an acrylonitrile polymer or copolymer. 
     2. Description of the Prior Art 
     An acrylonitrile polymer or copolymer has excellent gas-impermeability and transparency, and due to these properties it is used in the fields of receptacles for aerated drinks and casings for foods. In these fields, the receptacles and casings are exposed to an aqueous medium at high temperature for washing and sterilizing treatments. However, where the molded article of an acrylonitrile polymer or copolymer is exposed to an aqueous medium at a high temperature, there is the defect that transparency remarkably deteriorates, whereby appearance as a receptacle and casing is remarkably impaired. 
     Therefore, it has been strongly desired to obtain molded articles of acrylonitrile polymers or copolymers which do not remarkably deteriorate in transparency even though they are exposed to an aqueous medium at a high temperature. 
     It has further been desired to remove the water absorption whitening of molded articles of acrylonitrile polymers or copolymers to provide transparent molded articles thereof. 
     SUMMARY OF THE INVENTION 
     As a result of research to overcome the above defects, the inventor found that where at least one kind of specific salt is added to an aqueous medium in an amount such that the concentration is from about 5% by weight to the saturation amount thereof, the transparency of a molded article of an acrylonitrile polymer or copolymer can be maintained without the remarkable deterioration thereof, even though the molded article is exposed to the aqueous medium at a high temperature, and further found that where a nontransparent molded article of an acrylonitrile polymer or copolymer (due to the water-absorption whitening) is immersed in the above-mentioned salt-containing aqueous medium at a high temperature, the whitening can be removed whereby a transparent molded article can be obtained again. 
     Therefore, the present invention provides a method for eliminating the above defects. 
     One object of the present invention is to provide a method for preventing the water-absorption whitening of a molded article of an acrylonitrile polymer or copolymer by immersing the molded article into an aqueous medium containing at least one kind of salt. 
     Another object of the present invention is to provide a method for eliminating water-absorption whitening of a nontransparent molded article of an acrylonitrile polymer or copolymer (due to the water-absorption whitening) by immersing the nontransparent molded article into an aqueous medium containing at least one kind of salt. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the method of the present invention, there are the advantages that washing and sterilizing treatments of molded articles containing an acrylonitrile polymer or copolymer can be easily carried out in an aqueous medium at high temperature without deteriorating transparency and that the transparency of a non-transparent molded article due to the water-absorption whitening can be restored. 
     The acrylonitrile polymers used in the present invention include homopolymers of a nitrile-containing olefinic monomer, and the acrylonitrile copolymers used in the present invention include copolymers of at least about 50% by weight of the nitrile-containing olefinic monomer and up to about 50% by weight of at least one olefinically unsaturated comonomer. 
     The nitrile-containing olefinic monomer is a compound represented by the general formula ##STR1## , wherein R represents hydrogen, a halogen or an alkyl group having 1 to 4 carbon atoms. 
     Examples of the nitrile-containing olefinic monomer include acrylonitrile, α-chloroacrylonitrile, α-fluoroacrylonitrile, methacrylonitrile, ethacrylonitrile or the like. 
     Examples of the olefinically unsaturated comonomer include alkyl esters of acrylic acid and methacrylic acid, each having 1 to 12 carbon atoms in the alkyl moiety, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate or the like; a vinyl ester of a carboxylic acid having 2 to 12 carbon atoms such as vinyl acetate, vinyl propionate or the like; an alkyl vinyl ether having 1 to 12 carbon atoms such as methyl vinyl ether, ethyl vinyl ether, butyl vinyl ether or the like; an α-olefin having 2 to 8 carbon atoms such as ethylene, propylene, isobutylene, butene-1 or the like; a compound of the formula CH 2  ═ CHX (wherein X is a halogen or an aromatic group) such as vinyl chloride, styrene or the like; a compound of the formula ##STR2## (wherein R is a halogen or an alkyl group having 1 to 2 carbon atoms and X is a halogen or an aromatic group such as a phenyl group) such as vinylidene chloride, α-chlorostyrene, α-methylvinyl chloride, α-methyl styrene or the like; maleimide; a copolymerizable N-substituted maleimide such as N-phenyl maleimide, N-4-diphenyl maleimide, N-1-naphthyl maleimide or the like. 
     The method according to the present invention is effective not only on the above-described molded articles of an acrylonitrile polymer or copolymer but also on a composition prepared by blending 1 to 30% by weight of a synethetic rubber or a natural rubber with an acrylonitrile polymer or copolymer and also a composition prepared by grafting 1 to 30% by weight of the synthetic rubber or natural rubber with the acrylonitrile polymer or copolymer. 
     The synthetic or natural rubber include a butadiene polymer; a copolymer of butadiene and a monomer copolymerizable therewith (such as a copolymer of butadiene with acrylonitrile, an acrylic acid alkyl ester or methacrylic acid alkyl ester, or the like); a conjugated diene rubber ( such as isoprene, neoprene or the like); an olefinic rubber (such as a copolymer of ethylene and propylene, or the like); a chlorinated olefinic rubber; or the like. 
     Further, the method according to the present invention is also effective on composite products of the acrylonitrile polymer or copolymer and another polymer, such as a polyolefin composite film, as well as a molded article of the acrylonitrile polymer or copolymer alone. 
     The present invention is of general application to solid acrylonitrile-comprising materials as above discussed, and the molecular weight of materials treatable in accordance with the present invention is not limited in any specific fashion. Conventionally, materials treatable in accordance to the present invention (excluding graft copolymers) have a viscosity η sp/c of about 0.04 to about 0.8 1/g (in dimethylformamide at 30° C). 
     The salt used in the present invention is a halide (except for an iodide), a nitrate or an acetate of an element included in Groups I, II, III and VIII, Periods 1 to 4 of the Periodic Table (except for zinc). The salt must have a solubility in water at the temperature of use of at least 10% by weight. 
     Examples of the salt include NaCl, KCl, NaBr, MgCl 2  CaCl 2 , MgBr 2 , CaBr 2 , AlCl 3  AlBr 3 , FeCl 2 , FeCl 3 , NaNO 3 , KNO 3 , Mg(NO 3 ) 2 , Ca(NO 3 ) 2 , Al(N0 3 ) 3 , Fe(NO 3 ) 2 , Fe(N0 3 ) 3 , CH 3  COONa or the like. An iodide of such an element, such as NaI, or a zinc salt, such as ZnCl 2 , cannot be used in the method of the present invention since the molded article of the acrylonitrile polymer or copolymer swells or dissolves in an aqueous medium at high temperature containing those salts. 
     The salt added to the aqueous medium is not limited to one salt and a mixture of the salts can be used. 
     An amount of the salt added to the aqueous medium is from about 5% by weight to a saturation amount of the salt in water at the temperature of use, preferably the saturation amount or in the vicinity thereof. 
     In the case of preventing water-absorption whitening, such is generally conducted at the time of cooking or sterilizing the molded article itself or foods filled therein. Therefore, the parameters of treatment, such as the temperature of aqueous medium, the immersion time, etc., are determined by the cooking or sterilizing conditions. However, according to the method of the present invention, a temperature of the aqueous medium of from about 30° C to the boiling point of the aqueous medium is in general effective to obtain satisfactory effects. 
     In the case of eliminating water-absorption whitening, the temperature of the aqueous medium is generally from about 65° C to the boiling point, preferably from about 80° C to the boiling point. The immersion time varies depending upon the immersion temperature and the water-absorption amount. That is, the immersion time can be shortened as the immersion temperature becomes higher and the water-absorption amount in the molded article becomes smaller. 
     The method of the present invention is most conveniently performed at atmospheric pressure. Nothing prohibits the use of sub- or super-atmospheric pressure, but nothing is gained in the sense of increased process efficiency or superior product capabilities by utilizing such more complicated systems, and, accordingly, seldom will such ever be used. 
     In most instances, in the case of preventing water-absorption whitening the treatment is conducted for at most 30 days, while for eliminating water-absorption whitening, the treatment is generally conducted in a period of time of from about 1 minute to about 10 days, more preferably from 1 minute to 2 hours. 
     Further, the degree of restoration of the transparency varies depending upon the salt concentration and the immersion time. 
    
    
     The present invention will be illustrated more specifically by reference to the following examples, but it is to be understood that they are included merely for purposes of illustration and are not intended to be construed as limiting the scope of the present invention. 
     EXAMPLE 1 
     An acrylonitrile copolymer comprising 75% by weight of acrylonitrile and 25% by weight of methyl acrylate [viscosity η sp/c = 0.08 l/g (in dimethylformamide at 30° C)] was pressed by means of a compression machine at 200° C to obtain a sheet having a thickness of 1 mm. The sheet obtained was immersed in water containing a salt at the concentration as shown in Table I below at 90° C for 30 minutes. The haze value of the sheet 30 minutes after and 24 hours after the immersion treatment were measured according to the criteria of ASTM D1003-61. For the sake of comparison, water containing no salt was also used. 
     The results obtained are shown in Table 1 below. 
     
                                           Table 1                                 
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                        Haze Value (%)                                    
                Salt    Before                                            
                              30 Min. After                               
                                          24 Hours After                  
Run No.   Salt  Concentration                                             
                        Immersion                                         
                              Immersion Treatment                         
                                          Immersion Treatment             
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                (wt. %)                                                   
1. (Control)                                                              
           --    0      6.7   90.9        97.0                            
2. (Invention)                                                            
          NaCl  10      6.1   43.1        88.6                            
                15      6.8   19.1        46.6                            
                20      6.2   8.1         9.0                             
                Saturation                                                
                        6.1   8.1         9.0                             
3. (Invention)                                                            
          KCl   10      6.0   60.5        95.5                            
                20      6.8   28.7        66.1                            
                Saturation                                                
                        6.2   7.6         8.4                             
4. (Invention)                                                            
          NaBr  25      6.1   14.7        32.4                            
                Saturation                                                
                        6.1   8.3         9.5                             
5. (Invention)                                                            
          CaCl.sub.2                                                      
                 5      6.5   40.0        87.0                            
                10      6.8   11.2        18.7                            
                25      6.6   7.5         8.7                             
6. (Invention)                                                            
          MgBr.sub.2                                                      
                25      6.4   55.0        92.5                            
                Saturation                                                
                        6.5   9.0         10.5                            
7. (Invention)                                                            
          AlCl.sub.3                                                      
                10      6.5   49.8        92.2                            
                20      7.7   10.3        11.2                            
8. (Invention)                                                            
          NaNO.sub.3                                                      
                30      7.1   25.2        59.9                            
9. (Invention)                                                            
          Ca(NO.sub.3).sub.2                                              
                30      6.5   9.7         10.9                            
10.                                                                       
   (Invention)                                                            
          CH.sub.3 CO.sub.2 Na                                            
                50      6.5   8.5         9.7                             
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     EXAMPLE 2 
     The same procedure as in Example 1 was repeated except for using an acrylonitrile copolymer composition prepared by blending 20 parts by weight of a rubber component which was prepared by grafting 50 parts by weight of a monomer mixture of 60% by weight of acrylonitrile and 40% by weight of ethyl acrylate onto 50 parts by weight of a copolymer comprising 80% by weight of butadiene and 20% by weight of methyl acrylate, with 80 parts by weight of a copolymer comprising 75% by weight of acrylonitrile and 25% by weight of methyl acrylate. 
     The results obtained are shown in Table 2 below. 
     
                                           Table 2                                 
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                      Haze Value (%)                                      
         Kind of                                                          
              Concentration                                               
                      Before                                              
                            30 Min. After                                 
                                        24 Hours After                    
Run No.   Salt                                                            
              of Salt Immersion                                           
                            Immersion Treatment                           
                                        Immersion Treatment               
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              (wt.%)                                                      
  (Control)                                                               
          --   0      6.0   90.8        96.0                              
  (Invention)                                                             
         NaCl 20      6.5   8.1         9.2                               
              Saturation                                                  
                      6.7   8.0         9.0                               
  (Invention)                                                             
         CaCl.sub.2                                                       
              10      7.1   10.0        17.5                              
              20      6.8   8.0         9.1                               
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     EXAMPLE 3 
     The same procedure as in Example 1 was repeated except for using a saturated NaCl aqueous solution and changing the immersion treatment time. 
     The results obtained are shown in Table 3 below. 
     
                       Table 3                                                     
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Immersion Treatment                                                       
              Before     24 Hours after                                   
Time          Immersion  Immersion Treatment                              
______________________________________                                    
(hr.)                                                                     
0.5           6.1        9.0                                              
1             6.7        10.0                                             
2             6.3        9.7                                              
6             6.3        52.4                                             
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     EXAMPLE 4 
     Using a copolymer film comprising 75% by weight of acrylontrile and 25% by weight of methyl acrylate having a thickness of 20 μ ]viscosity η sp/c = 0.08 l/g (in dimethylformamide at 30° C)], a polyethylene film having a thickness of 50 μ and a polypropylene film having a thickness of 70 μ, three laminate films were prepared using an urethane adhesive as an adhesive (Nipporan 3002-Colonate L, manufactured by Nippon Polyurethane Industry Co., Ltd.). 
     Each of the laminate films obtained was treated under the conditions shown in Table 3 below. The laminate films thus treated were allowed to stand for one week at room temperature and the transparency thereof was visually judged. 
     For the sake of comparison, a copolymer film of 75% by weight of acrylonitrile and 25% by weight of methyl acrylate having a thickness of 20 μ was treated as described and judged. 
     The results obtained are shown in Table 4 below. 
     
                                           Table 4                                 
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                        Control         Invention                         
                        Water           Saturated NaCl aqueous solution   
Sample Tested   75° C 30min.                                       
                        85° C 30 min.                              
                                95° C 30 min.                      
                                        75° C 30                   
                                                85° C 30           
                                                        95° C 30   
                                                        min.              
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                non-    non-    non-                                      
   I*           transparent                                               
                        transparent                                       
                                transparent                               
                                        transparent                       
                                                transparent               
                                                        transparent       
Polyethylene/I/polyethylene                                               
                &#34;       &#34;       &#34;       &#34;       &#34;       &#34;                 
Polypropylene/I/polypropylene                                             
                slightly                                                  
                non-transparent                                           
                        &#34;       &#34;       &#34;       &#34;       &#34;                 
Polyethylene/I/polypropylene                                              
                non-transparent                                           
                        &#34;       &#34;       &#34;       &#34;       &#34;                 
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 Note                                                                     
 *A copolymer film of 75% by weight of acrylonitrile and 25% by weight of 
 methylacrylate having a thickness of 20 μ.                            
 
    
     EXAMPLE 5 
     The sheet used in Example 1 was immersed under the immersion conditions shown in Table 5 below to obtain a non-transparent sheet. The water-absorption amount and the Haze value of the non-transparent sheet obtained were measured and are shown in Table 5 below. 
     The non-transparent sheet was again immersed in a saturated NaCl aqueous solution under the immersion conditions shown in Table 5 below. The Haze value of the sheet after reimmersion was measured and is shown in Table 5 below. 
     
                                           Table 5                                 
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Before Reimmersion            After Reimmersion                           
                  Water                 Reim- Reim- Haze Value 10 Min.    
Immersion                                                                 
      Immersion                                                           
            Immersion                                                     
                  Absorption                                              
                        Haze  Reimmersion                                 
                                        mersion                           
                                              mersion                     
                                                    After Re-             
Medium                                                                    
      Temp. Time  Amount                                                  
                        Value Medium    Temp. Time  immersion             
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                                                    Treatment             
      (° C)                                                        
            (min.)                                                        
                  (wt.%)                                                  
                        (%)             (° C)                      
                                              (min.)                      
                                                    (%)                   
Water 90    150 4.9                                                       
                  96.2  Saturated NaCl                                    
                              90        60    57.2                        
                              aqueous soln.                               
                                &#34;       90    120   26.7                  
                                &#34;       90    150   18.3                  
Water 90    30    2.2   86.2  Saturated NaCl                              
                                        90    30     9.1                  
                              aqueous soln.                               
Water 90    30    0.7*  96.0* Saturated NaCl                              
                                        90    10    23.0                  
                              aqueous soln.                               
                                &#34;       90    20    10.8                  
Water 90    30    0.7*  96.0* Saturated NaCl                              
                                        80    30    18.0                  
                              aqueous soln.                               
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 Note-                                                                    
 *Values of the sample which was dried at room temperature for one week   
 after the first immersion treatment.                                     
 
    
     As is apparent from the results in Examples 1 to 4, the use of an aqueous medium containing a specific salt according to the present invention provides excellent effects in preventing water-absorption whitening as compared with the use of water only. 
     It is further apparent from the results in Example 5 that the use of an aqueous medium containing specific salts according to the present invention can markedly improve the haze value of sheet as compared with the use of water only. 
     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.