Process for producing metal-boehemite laminates
This invention relates to a process for the formation of a laminate of an aluminum sheet or foil with a polyolefin film and more particularly to a process for forming such a laminate particularly suitable as heat sealable material for package of foodstuffs without the use of a binder for lamination, the laminate being very advantageous from the view-point of sanitation. 
     There have heretofore been marketed various laminates of aluminum sheets or foils with various synthetic resin films and, among others, laminates of aluminum sheets or foils with polyolefin films have recently been particularly noted and in increasingly great demand with the development of package of foodstuffs. 
     However, there was raised a problem as to the bond strength effected between a polyolefin film and a metal such as an aluminum sheet or foil (the sheet and foil being hereinafter referred to simply as &#34;foil&#34;) since the polyolefin film has no polar groups. 
     In an attempt to effect a satisfactory bond between an aluminum foil and a polyolefin film, many efforts have been made to develop binders useful for bonding the aluminum foil to the polyolefin film, treat the polyolefin film for improvement thereof in receptivity for the aluminum foil, treat the aluminum foil for improvement thereof in receptivity for the polyolefin film, effect such development and treatments in combination and find other suitable means for the same purpose as above. 
     Treatments heretofore known for improving a polyolefin film and aluminum foil in bonding to each other include, as treatments for aluminum foil, a mechanical treatment with sand blast or the like, an etching treatment with an acid or alkali and a chemical film-forming treatment such as a treatment by anodic oxidation or with phosphoric or chromic acid, and include, as treatments for a polyolefin film, a treatment with air, oxygen or ozone, a treatment with radiation, a treatment with sulfuric acid-chromic acid, a treatment with maleic or like acid for graft copolymerization, a treatment by corona discharge, a treatment with flame, a treatment by ultraviolet radiation and a treatment for incorporating an inorganic filler in the film. 
     However, even the use of these known treatments in combination did not result in the production of laminates having satisfactory bond strength and, therefore, binders had to be used for lamination. The thus-produced laminates in which the binder is used are disadvantageous in that the binder will melt out of the laminates if they are made into bags and subjected to a high temperature treatment for sterilization or the like. The term &#34;binder&#34; in the specification is intended to be synonymous with &#34;adhesive&#34;. 
     Various studies were made by the present inventors in attempts to find a process for bonding an aluminum foil and a polyolefin film securely to each other without the use of a binder and, as a result of their studies, it has been found that even if an aluminum foil having been subjected to known boehmite treatment (such foil being hereinafter referred to as &#34;boehmite-treated foil&#34;) is attempted to be laminated with a polyolefin film at a high temperature under pressure in a long time, a laminate having sufficient bond strength will not be obtained (a secure bond will not be effected between the boehmite-treated aluminum foil and the polyolefin film depending upon the kind of the polyolefin whereby are raised problems as to the operation of lamination and use of the laminates produced). 
     As a result of their further studies, the present inventors have also found that if a boehmite-treated aluminum foil is laminated with a polyolefin film having previously been subjected to oxidation treatment, a satisfactory laminate having high bond strength (a laminate in which the aluminum foil and the polyolefin film are securely bonded to each other) will be produced under heat and pressure in a short time. Such laminates may be used as material for packaging general goods and medical supplies, particularly for packaging foodstuffs. 
     The boehmite treatment effected in the practice of this invention may comprise (1) immersing an aluminum foil in a distilled water or ion exchange water at not lower than 60° C. to which a small amount of ammonia water, triethanolamine or the like may be added, (2) treating an aluminum foil in steam at 100°-140° C. or (3) subjecting an aluminum foil to any of other known methods for boehmite treatment. 
     The polyethylene films which may be used in the invention include commercially available films such as high-density, medium-density and low-density polyethylene films, and stretched and non-stretched polypropylene films. 
     The corona discharge treatment used herein may be a usual one. 
     The lamination under heat and pressure (or the thermal pressure lamination) according to this invention will provide a laminate which has high bond strength without a binder and is suitable as material particularly for packaging foodstuffs, the laminate consisting of an oxidation-treated polyolefin film laminated to a boehmite- or pseudoboehmite-treated aluminum foil with the treated surface of the foil and that of the film facing to each other. 
     These and other objects will be apparent from the following description. 
     The aluminum foils (including sheets as previously defined) used herein are not required to be of special quality but may be commercially available untreated ones. 
     The boehmite or pseudoboehmite treatment used in this invention may be effected by the use of any one of heretofore known various methods. For example, it may be effected by immersing an aluminum foil in distilled water or ion exchange water at not lower than 60° C. or by blowing steam (at 100°-140° C.) against an aluminum foil. The time required for the boehmite treatment varies depending partly on a temperature for the treatment but it ranges from about 30 seconds to several hours, with from two minutes to 30 minutes being preferred for industrial purposes. In the practice of boehmite treatment used in this invention, it is not always necessary to make a perfect boehmite layer of an aluminum foil surface; on the other hand, it is enough to make the foil surface into pseudoboehmite even if the pseudoboehmite is one which has been formed in less than several minutes, since even the formation of such pseudoboehmite will result in the production of laminates having sufficient bond strength which are suitable for the purpose of this invention. For this it is considered that the process of this invention is a very advantageous one for industrial use. The term &#34;boehmite and/or pseudoboehmite&#34; is hereinafter referred to simply as &#34;boehmite&#34; for brevity. In the boehmite treatment, said distilled water or ion exchange water may be incorporated with ammonia or an amine such as triethanolamine to improve the resulting boehmite-treated aluminum foil in corrosion resistance. 
     The polyolefin films which may be used in this invention include polyethylene films, preferably high-density polyethylene films, and polypropylene films, preferably unstretched polypropylene films. 
     The oxidation treatment for these polyolefin films may be at least one of a corona discharge treatment, an ultraviolet radiation treatment, a glow discharge treatment, an electron beam radiation treatment, an ozone treatment, a flame treatment and a sulfuric acid-chromic acid treatment, among which the corona discharge treatment is particularly advantageous for the purpose of this invention since it is easy to carry out with safety, dose not cause aging of the polyolefin films, does not raise problems as to disposal of waste water and provides oxidation-treated polyolefin films capable of being securely bonded to boehmite-treated aluminum foils. The use of the corona discharge and ultraviolet radiation treatments in combination is very advantageous industrially since it will result in the production of satisfactory laminates under heat and pressure in a short time. 
     In the lamination of the boehmite-treated aluminum foil with the oxidation-treated polyolefin film, there are two cases where the treated polyolefin film is used in non-molten state and molten state, respectively. In the form case, commercially available polyolefin films may be used (after oxidation-treated if not so treated), and particularly oxidation treated high-density polyethylene and unstretched polypropylene films may preferably be used. It is advantageous that these treated polyethylene and polypropylene films may be laminated to the boehmite-treated aluminum foils by the use of a comparatively simple thermal pressure laminating apparatus and it is also desirable that the laminates so obtained may be used as material for packaging foodstuffs since they are excellent in heat resistance. 
     In the latter case, it is advantageous as compared with the former case that a better bond may be effected in the production laminates, while it is disadvantageous that the polyolefin films may, in some cases, be degraded in quality and that a complicated thermal pressure lamination apparatus be required for lamination. Thus, it may be determined depending on the use of the resulting laminates which case to select. 
     In the lamination of the boehmite-treated aluminum foil with the oxidation-treated polyolefin film according to this invention, a hot press or rolls for thermal pressure lamination (hereinafter referred to simply as &#34;lamination rolls&#34;) may be used with the latter being preferably used for industrial use. It is desirable that the lamination rolls be operated under the conditions that the temperature of the surface of the rolls is in the range of 140°-240° C., the pressure exerted by the rolls is in the range of 1-10 kg/cm 2  (in terms of a plane pressure) and the peripheral velocity of the rolls is in the range of 1-50 m/min. 
    
    
     As apparatus for carrying out this invention, conventional apparatus for lamination using a binder under heat and pressure may be used although the step of applying a binder is omitted; for example, apparatus for extrusion lamination, for hot melt lamination or for dry lamination may be used. Such apparatus will be illustrated by reference to the accompanying drawing in which: 
     FIG. 1 illustrates a schematic view of an apparatus for carrying out a process of this invention; FIGS. 2(a) and 2(b) show a cross-sectional view of a conventional laminate; 
     FIGS. 3(a) and 3(b) show a cross-sectional view of a laminate produced by the process of this invention; 
     FIG. 4 shows a cross-sectional view of a laminate of this invention used for packaging; and 
     FIG. 5 shows laminates of this invention in doubled form. 
    
    
     Referring now to FIG. 1, a polyolefin film 1 is subjected to at least one of oxidation treatments such as corona discharge treatment 2 and ultraviolet radiation treatment 3 and then passed to lamination rolls consisting of a heating roll 4 and pressure roll 5, while an aluminum foil 6 is subjected to boehmite treatment and passed, through heating rolls 8 and 8&#39; if desired, to said lamination rolls where the boehmite-treated aluminum foil is laminated with the oxidation-treated polyolefin. If desired, the laminate so obtained is then subjected to ultraviolet radiation treatment 3&#39; with the polyolefin film side facing to the radiation source or cooled by passing through cooling rolls 9 and 9&#39;. 
     The use of the new laminates produced by the process of this invention as well as comparison of the new laminates with conventional (heretofore commercially available) ones will be detailed by reference to the accompany drawing. 
     As shown in FIG. 2(a), a conventional laminate consists of a polyolefin film 10 and an aluminum foil 12, laminated with each other with the use of a binder 11 therebetween and, as shown in FIG. 2(b), said laminate may be covered with another film 13 such as a polyester film or polyamide film with the use of a binder 11&#39; such as an isocyanate type adhesive between the foil 12 and other film 13 in order to increase the strength of the laminate. 
     As shown in FIG. 3(a), a laminate produced by the process of this invention consists of an oxidation-treated polyolefin film 10&#39; and a boehmite-treated aluminum foil 12&#39;, laminated with each other without the use of a binder therebetween. As shown in FIG. 3(b), the laminate is usually further laminated with a polyester or polyamide film 13 with the use of a binder between the foil 12&#39; and the film 13 for use as packaging material. 
     As indicated in FIG. 4, the packaging material as shown in FIG. 3(b) is heat sealed at the ends A and A&#39; with the polyolefin film 10&#39; facing to goods 14 such as foodstuffs, contained in the packaging material prior to the heat sealing. 
     This invention will be better understood by the following Examples and Comparative examples, in which oxidation treatments were effected as follows: 
     
         ______________________________________                                    
Corona discharge treatment:                                               
                  70-80 Kw                                                
                  4000 cycle/sec                                          
                  120 m/min (Film travelling                              
                  speed)                                                  
Ultraviolet radiation treatment:                                          
                  10 2-Kw mercury lamps                                   
                  Films passed at 120 m/min                               
                  10 cm below each of the lamps                           
Electron beam treatment:                                                  
                  2 megarad, an instant                                   
                  (about 1 second)                                        
Glow discharge treatment:                                                 
                  About 7Kw, 50m/min (Film                                
                  travelling speed)                                       
Flame treatment:  Films passed over the flame                             
                  of a gas burner in 5/10,000 sec.                        
Sulfuric acid-chromic acid treatment:                                     
                    Sulfuric acid  1500 parts                             
Composition         Potassium bichromate                                  
                                    75 parts                              
(parts by weight)   Water           120 parts                             
Immersed for just a moment (about 1 second).                              
0zone treatment:                                                          
             Films passed at 100 m/min 10 cm below                        
             an ozone generator capable of generating                     
             0.5 kg ozone/hr.                                             
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     Comparative example 1 
     Aluminum foils (untreated, 15μ thick, soft, produced by Sun Aluminum Co., Ltd.), and treated aluminum foils obtained respectively by (1) anodically oxidizing an aluminum foil in a 15% aqueous solution of sulfuric acid for one minute (hereinafter called &#34;treatment (1))&#34; (2) anodically oxidizing an aluminum foil in 3% aqueous solution of oxalic acid for one minute (hereinafter called &#34;treatment (2)&#34;, (3) immersing an aluminum foil in an aqueous solution at 60° C. of 65 g HNO 3 , 80 g NH 4  HF 2  and 0.1 g Pb(NO 3 ) 2  in one liter of water for two seconds (hereinafter called &#34;treatment (3)&#34;) and (4) immersing an aluminum foil in aqueous solution at 100° C. of 500 g H 3  PO 4  and 10 g HNO 3  in one liter of water for 6 seconds (hereinafter called &#34;treatment (4)&#34;, were laminated respectively with untreated, corona discharge-treated, ultraviolet radiation-treated and flam-treated LDPE (low-density polyethylene films), HDPE (high-density polyethylene films) and CPP (unstretched polypropylene films) by the use of lamination rolls heated respectively to 140° C., 160° C., 180° C., 200° C. and 220° C. and rotated at a peripheral velocity of 1 m/min under a pressure of 2 kg/cm 2  in terms of a plane pressure, thereby to produce laminates. The laminates so produced were measured for peel strength by the use of a peel strength tester (produced under the trademark of TENSILON by Toyo Sokki Co., Ltd., Japan) and the results obtained are shown in Tables 1-3. The starting polyolefin films used herein were LDPE, 60-70μ thick, produced under the trademark of B&#39;-5 by Tamapori Co., Ltd., HDPE, 60-70μ thick, produced under the trademark of DT-2 by Tamapori Co., Ltd., and CPP, 60-70μ thick, produced under the trademark of TORAYPHANE by Toray Co., Ltd. 
     In the following Tables, the term &#34;treated&#34; means &#34;treated on at least one side of a foil or film&#34;. The numeral values represent &#34;peel strength in g/cm&#34; and the symbol &#34;-&#34; indicates &#34;hardly bonded&#34; in the Tables. 
     
                                           TABLE 1                                 
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Treatment                                                                 
on    Treatment Lamination temp.                                          
aluminum                                                                  
      on LDPE   140° C.                                            
                    160° C.                                        
                        180° C.                                    
                            200° C.                                
                                220° C.                            
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      Untreated --  --  --  --  --                                        
      Corona discharge                                                    
                --   60  70 180 190                                       
Untreated                                                                 
      Ultraviolet radiation                                               
                --  --   60  70 170                                       
      Flame     --  --   60  70 180                                       
      Untreated --  190 200 240 250                                       
Treatment                                                                 
      Corona discharge                                                    
                --  250 260 270 280                                       
(1)   Ultraviolet radiation                                               
                --  230 250 250 260                                       
      Flame     --  230 240 240 260                                       
      Untreated --  190 200 240 250                                       
Treatment                                                                 
      Corona discharge                                                    
                --  240 260 270 280                                       
(2)   Ultraviolet radiation                                               
                --  240 240 240 260                                       
      Flame     --  230 240 240 260                                       
      Untreated --  --  --  230 230                                       
Treatment                                                                 
      Corona discharge                                                    
                --   90 200 260 270                                       
(3)   Ultraviolet radiation                                               
                --  --  180 200 230                                       
      Flame     --  --  180 200 230                                       
      Untreated --  --  --  230 230                                       
Treatment                                                                 
      Corona discharge                                                    
                --   90 200 260 270                                       
(4)   Ultraviolet radiation                                               
                --  --  180 200 230                                       
      Flame     --  --  180 200 230                                       
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                                           TABLE 2                                 
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Treatment                                                                 
on    Treatment Lamination temp.                                          
aluminum                                                                  
      on HDPE   140° C.                                            
                    160° C.                                        
                        180° C.                                    
                            200° C.                                
                                220° C.                            
__________________________________________________________________________
      Untreated --  --  --  --  --                                        
Untreated                                                                 
      Corona discharge                                                    
                --  --  --   50  50                                       
      Ultraviolet radiation                                               
                --  --  --  --  --                                        
      Flame     --  --  --  --  --                                        
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  100 100 130 130                                       
(1)   Ultraviol;et radiation                                              
                --  --  100 100 100                                       
      Flame     --  --  100 100 100                                       
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  100 100 130 130                                       
(2)   Ultraviolet radiation                                               
                --  --  100 100 110                                       
      Flame     --  --  100 100 110                                       
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  --   60  60  70                                       
(3)   Ultraviolet radiation                                               
                --  --   50  50  60                                       
      Flame     --  --   50  50  60                                       
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  --   50  60  80                                       
(4)   Ultraviolet radiation                                               
                --  --   50  50  50                                       
      Flame     --  --   50  60  60                                       
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                                           TABLE 3                                 
__________________________________________________________________________
Treatment                                                                 
on    Treatment Lamination temp.                                          
aluminum                                                                  
      on CPP    140° C.                                            
                    160° C.                                        
                        180° C.                                    
                            200° C.                                
                                220° C.                            
__________________________________________________________________________
      Untreated --  --  --  --  --                                        
untreated                                                                 
      Corona discharge                                                    
                --  --  --   50  50                                       
      Ultraviolet radiation                                               
                --  --  --   30  30                                       
      Flame     --  --  --  --  --                                        
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  100 100 130 130                                       
(1)   Ultraviolet radiation                                               
                --  --  110 110 110                                       
      Flame     --  --  100 100 110                                       
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  100 100 130 130                                       
(2)   Ultraviolet radiation                                               
                --  --  100 110 120                                       
      Flame     --  --  100 100 120                                       
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  --   50  60  80                                       
(3)   Ultraviolet radiation                                               
                --  --   60  60  70                                       
      Flame     --  --   50  50  60                                       
      Untreated --  --  --  --  --                                        
Treatment                                                                 
      Corona discharge                                                    
                --  --   50  60  80                                       
(4)   Ultravioilet radiation                                              
                --  --   50  50  70                                       
      Flame     --  --   50  60  70                                       
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     Comparative example 2 
     Aluminum foils were subjected to boehmite treatment by being immersed in ion exchange (deionized) water at 100° C. for 10, 5, 3 and 2 minutes, respectively. 
     The treated aluminum foils were then laminated on the boehmite-treated surface with LDPE, HDPE and CPP at 140° C., 160° C., 180° C., 200° C. and 220° C., respectively, by the use of lamination rolls rotating at a peripheral velocity of 1 m/min under a pressure of 2 kg/cm 2  in terms of a plane pressure thereby to obtain laminates. Each of the laminates so obtained was measured for peel strength with the results being shown in Table 4. 
     
                       TABLE 4                                                     
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Time                                                                      
required                                                                  
for                                                                       
boehmite                                                                  
treatment                                                                 
        Poly- -of                                                         
                 Olefin Lamination temp.                                  
aluminum                                                                  
        used     140° C.                                           
                         160° C.                                   
                               180° C.                             
                                     200° C.                       
                                           220° C.                 
______________________________________                                    
10 min. L D P E  240     230   280   330   350                            
5 min.           250     240   270   300   330                            
3 min.           180     220   260   290   300                            
2 min.           110     170   240   290   300                            
10 min. H D P E  160     160   170   170   190                            
5 min.            90     160   170   170   170                            
3 min.           120     150   150   160   160                            
2 min.           120     120   140   160   160                            
10 min. C P P    --      200   210   210   220                            
5 min.           --      200   200   210   220                            
3 min.           --      180   180   180   190                            
2 min.           --      140   150   160   170                            
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     EXAMPLE 1 
     Aluminum foils were immersed in ion exchange water (deionized water) at 100° C. for 10, 5, 3 and 2 minutes in order to effect boehmite treatment thereon, respectively. The aluminum foils so treated were laminated with corona discharge-treated LDPE, HDPE and CPP with the treated surface of the foil facing to the treated surface of the film at 140° C., 160° C., 180° C., 200° C. and 220° C., respectively, by the use of lamination rolls rotating at a peripheral velocity of 1 m/min under a pressure of 2 kg/cm 2  in terms of a plane pressure thereby to obtain laminates. The laminates so obtained were measured for peel strength with the results being indicated in the following Table 5. 
     
                       TABLE 5                                                     
______________________________________                                    
Time                                                                      
required                                                                  
for                                                                       
boehmite                                                                  
treatment                                                                 
        Poly-                                                             
of      olefin   Lamination temp.                                         
aluminum                                                                  
        used     140° C.                                           
                         160° C.                                   
                               180° C.                             
                                     200° C.                       
                                           220° C.                 
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10 min. L D P E  390     470   510   530   530                            
5 min.           370     370   380   420   430                            
3 min.           310     330   370   450   460                            
2 min.           300     320   330   340   350                            
10 min. H D P E  520     600   620   630   630                            
5 min.           510     580   600   620   620                            
3 min.           320     500   510   540   550                            
2 min.           230     360   410   420   430                            
10 min. C P P     60     580   630   640   630                            
5 min.            70     600   640   630   640                            
3 min.            60     530   530   590   600                            
2 min.            60     370   460   470   480                            
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     EXAMPLE 2 
     Aluminum foils were subjected to boehmite treatment by immersing them in ion exchange water at 100° C. for 3 minutes. The aluminum foils so treated were then laminated respectively with corona discharge-treated and ultraviolet radiation-treated LDPE, HDPE and CPP with the treated surface of the foil facing to the treated surface of the film, in the same manner as in Example 1 thereby yielding laminates. The laminates so yielded were then measured for peel strength with the results being indicated in Table 6. 
     
                       TABLE 6                                                     
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Polyolefin                                                                
        Lamination temp.                                                  
used    140° C.                                                    
                 160° C.                                           
                          180° C.                                  
                                 200° C.                           
                                        220° C.                    
______________________________________                                    
L D P E 360      380      390    480    490                               
H D P E 350      530      550    580    580                               
C P P   130      580      580    630    640                               
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     EXAMPLE 3 
     Aluminum foils were subjected to boehmite treatment by immersing them in ion exchange water at 100° C. for 3 minutes. The boehmite-treated aluminum foils were laminated respectively with corona discharge-treated LDPE, HDPE and CPP with the treated surface of the foil and that of the film facing to each other, under the same heat and pressure as in Example 1 to yield laminates which were then subjected to ultraviolet radiation, thereby to obtain final laminates the peel strength of each of which is shown in Table 7. 
     
                       TABLE 7                                                     
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Polyolefin                                                                
        Lamination temp.                                                  
used    140° C.                                                    
                 160° C.                                           
                          180° C.                                  
                                 200° C.                           
                                        220° C.                    
______________________________________                                    
H D P E 500      700      700    740    740                               
C P P   380      630      730    780    800                               
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     EXAMPLE 4 
     Aluminum foils were subjected to boehmite treatment by immersing them in ion exchange water at 100° C. for 3 minutes. The boehmite-treated foils were then laminated respectively with corona discharge-treated HDPE and CPP with the treated surface of the foil and that of the film facing to each other, at 180° C. by the use of lamination rolls rotating respectively at peripheral velocities of 1 m/min, 2 m/min and 30 m/min under a pressure of 2 kg/cm 2  in terms of a plane pressure thereby to yield laminates the peel strength of each of which is shown in Table 8. 
     
                       TABLE 8                                                     
______________________________________                                    
Polyolefin                                                                
         Peripheral velocity of roll                                      
used     2m/min      3m/min      30m/min                                  
______________________________________                                    
H D P E  510         510         470                                      
C P P    530         530         500                                      
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     EXAMPLE 5 
     Aluminum foils were subjected to boehmite treatment by immersing in ion exchange water at 100° C. for 5 minutes. The aluminum foils so treated were then laminated with ultraviolet radiation-treated, flame-treated and glow discharge-treated HDPE and CPP with the treated surface of the foil and that of the film facing to each other, respectively, in the same manner as in Example 1 thereby to obtain laminates the peel strength of each of which is shown in Table 9. 
     
                                           TABLE 9                                 
__________________________________________________________________________
Polyolefin                                                                
       Treatment Lamination temp.                                         
used   on polyolefin                                                      
                 140° C.                                           
                     160° C.                                       
                         180° C.                                   
                             200° C.                               
                                 220° C.                           
__________________________________________________________________________
       Ultraviolet radiation                                              
                 300 330 380 390 400                                      
H D P E                                                                   
       Flame     310 320 370 390 390                                      
       Glow discharge                                                     
                 320 340 380 380 400                                      
       Ultraviolet radiation                                              
                 --  320 380 400 410                                      
C P P  Flame     --  320 370 380 390                                      
       Glow discharge                                                     
                 --  350 370 390 410                                      
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     EXAMPLE 6 
     Aluminum foils were subjected to boehmite treatment by immersing them in ion exchange water at 100° C. for 3 minutes. The boehmite-treated aluminum foils were then laminated with corona discharge-treated HDPE and CPP with the treated surface of the foil and that of the film facing to each other, respectively, in the same manner as in Example 1, thereby obtaining laminates having the respective peel strengths as shown in Table 10. 
     
                       TABLE 10                                                    
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Polyolefin                                                                
        Lamination temp.                                                  
used    140° C.                                                    
                 160° C.                                           
                          180° C.                                  
                                 200° C.                           
                                        220° C.                    
______________________________________                                    
H D P E 320      500      500    540    540                               
C P P    70      590      640    640    640                               
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     Some of the laminates so obtained were immersed in boiling water for 60 minutes and then measured for peel strength in order to find changes in peel strength. The remaining laminates were made into bags which were packed, in vacuo, respectively with water, a mixture of water and a salad oil in the ratio of 1:1 and curry. The bags so packed were allowed to stand in steam under pressure for 50 minutes, cooled, visually observed and then measured for peel strength with the results being indicated in Table 11. 
     
                       TABLE 11                                                    
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Polyethylene used                                                         
             Lamination temp.                                             
and Test condition                                                        
             140° C.                                               
                     160° C.                                       
                             180° C.                               
                                   200° C.                         
                                         220° C.                   
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H D P E                                                                   
Boiling water test                                                        
             320     500     500   540   540                              
Packed with water                                                         
             300     480     480   520   530                              
Packed with water                                                         
and salad oil                                                             
             290     440     440   480   490                              
Packed with curry                                                         
             300     460     460   510   520                              
C P P                                                                     
Boiling water test                                                        
              70     590     640   640   640                              
Packed with water                                                         
             --      570     620   620   620                              
Packed with water                                                         
and salad oil                                                             
             --      530     580   590   590                              
Packed with curry                                                         
             --      560     610   610   610                              
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     EXAMPLE 7 
     Laminates were produced in the same manner as in Example 6. The laminates so produced were measured for permeability to oxygen by the use of an oxygen permeability tester (produced under the trademark 0 x  -Tran 100 type by Nissei Sangyo Co., Ltd.) with the result that each of them exhibited 0 cc/m 2  /24 hr atm. 
     EXAMPLE 8 
     Laminates were prepared in the same manner as in Example 6. The laminates so prepared were measured for permeability to moisture by the use of an infra-red automatic moisture permeability tester (produced under the trademark of IRD-2C type by Nissei Sangyo Co., Ltd.) with the result that each of them exhibited 0 g/m 2  /24 hr atm.