Film process of making and antiblocking
This is a division of application Ser. No. 07/357,825, filed May 26, 1989 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an anti-blocking agent for plastic film, which is useful for producing antiblocking plastic films having an excellent anti-blocking property and lubricity, and useful as a food-packing, fiber-packing, and other industrial post materials. 
     (2) Description of the Related Arts 
     Usually, the conventional antiblocking agent for plastic films comprises silica, but these plastic films containing silica particles as an antiblocking agent are disadvantageous in that the resultant anti-blocking plastic films exhibit (1) a reduced transparency, (2) less glossiness, (3) a lower scratch resistance, and (4) an unsatisfactory color. Although attempts were made, before the present invention, to find a new antiblocking agent equal to silica, these attempts were not successful, and thus silica is still used as the antiblocking agent. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an antiblocking agent for plastic films, which has the same antiblocking activity as a conventional antiblocking agent consisting essentially of silica particles, and enables the resultant antiblocking plastic films to exhibit a satisfactory transparency, glossiness, scratch resistance, and color. 
     The above-mentioned object can be obtained by the antiblocking agent of the present invention for plastic films, which comprises fibrous magnesium oxysulfate particles. 
     The fibrous magnesium oxysulfate particles consist of a compound represented generally by the formula: 
     
         MgSO.sub.4.5Mg(OH).sub.2.3H.sub.2 O 
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The fibrous magnesium oxysulfate particles usable for the present invention are in the form of a white fine powder, preferably consisting of needle-shaped crystals, and preferably having a true specific gravity of 2.3, a bulk density of 0.1 g/cm 3  or less, more preferably 0.08 g/cm 3  or less, a particle length of 10 to 100 μm, more preferably 10 to 50 μm, a particle thickness of less than 1 μm, more preferably less than 0.5 μm, and a specific surface area of less than 10 m 2  / g, more preferably less than 8 m 2  /g, determined by the BET method. 
     The fibrous magnesium oxysulfate particles can be produced by, for example, a method in which magnesium oxide or magnesium hydroxide particles are dispersed in an aqueous solution of magnesium sulfate and then the aqueous solution dispersion is heated, or another method in which magnesium hydroxide particles are dispersed in a sulfuric acid aqueous solution and the dispersion is heated. Japanese Unexamined Patent Publication No. 56-149318 concretely discloses specific processes for producing the fibrous magnesium oxysulfate particles and properties of the products. 
     In the application of the antiblocking agent of the present invention, preferably the fibrous magnesium oxysulfate particles are mixed, in an amount of 200 to 10,000 ppm, more preferably 500 to 3,000 ppm, into a matrix consisting essentially of a thermoplastic resin. 
     When the amount of the antiblocking agent in the plastic resin matrix is too small, the resultant plastic resin film exhibits an unsatisfactory antiblocking activity, and when the content of the antiblocking agent in the matrix is too large, the resultant plastic resin film exhibits a saturated antiblocking activity and reduced physical properties. 
     The plastic resin to be mixed with the antiblocking agent of the present invention is not restricted to thermoplastic resins of a specific type and having a specific molecular weight, and preferably, the thermoplastic resin is selected from polyamide resins, polyester resins, polypropylene resins, polyethylene resins, polyvinylidene chloride resins, ethylene-vinyl acetate copolymer resins, copolymer resins of two or more of the above-mentioned monomers and modified polymer resins of the above-mentioned polymers and copolymers. 
     Preferably, the plastic polymer resin is selected from polyolefin resins, for example, polypropylene and polyethylene homopolymer resins and ethylene-propylene copolymer resins. 
     The antiblocking agent of the present invention can be incorporated into the plastic resin matrix by any known mixing method, for example, melt kneading, melt extruding, and rolling. Also, the mixture of the plastic resin matrix with the antiblocking agent can be formed into a film by any shaping method, for example, by an extruding method or an inflation method. 
     The plastic film containing the antiblocking agent of the present invention preferably has a thickness of 10 to 200 μm. 
     Also, the plastic film containing antiblocking agent of the present invention can further contain an additive comprising at least one of antioxidants, ultraviolet ray-absorbing agents, coloring materials, slipping agents and other types of antiblocking agents, as long as the additive does not cause a reduction or loss of the effect of the antiblocking agent of the present invention. 
     SPECIFIC EXAMPLES 
     The present invention will be further explained by way of the following examples, which are representative only and do not restrict the scope of the present invention. 
    
    
     EXAMPLE 1 AND COMPARATIVE EXAMPLES 1 and 2 
     In Example 1, a propylene homopolymer powder having a melt flow index of 8.0 g/10 min was mixed with 1,000 ppm of an antioxidant consisting of BHT (2,6-di-t-butylhydroxytoluene), 900 ppm of a slipping agent consisting of erucinic acid amide, and 2,000 ppm of an antiblocking agent of the present invention consisting of fibrous magnesium oxysulfate particles available under the trademark of MOSHAIJI and made by UBE INDUSTRIES CO., in the form of needle-shaped crystals having a length of 15 μm, a thickness of 0.3 μm, and a specific surface area of 8 m 2  / g, using a Henschel mixer (made by Mitsui-Miike Co.), and the resultant mixture was melt-extruded at a temperature of 220° C. through a pellet-forming extruder. 
     The resultant pellets were melt-shaped into a tubular film by using a water-cooled inflation film-forming machine having an extrusion nozzle diameter of 45 mm, under the following conditions. 
     
         ______________________________________                                    
Temperature distribution of                                               
the film-forming machine                                                  
Portion     C.sub.1 C.sub.2 C.sub.3                                       
                                 Adapter Die                              
______________________________________                                    
Temperature (°C.)                                                  
            180     200     200  220     200                              
______________________________________                                    
 Water temperature: 20° C.                                         
 Taking up speed: 35 m/min                                                
 
    
     The resultant polypropylene film had a thickness of 30 μm and a flattened tube width of 200 mm. 
     The antiblocking property of the resultant tubular film was tested by opening the tubular film and evaluating the resultant film based on the following five classes. 
     
         ______________________________________                                    
Class     Property                                                        
______________________________________                                    
5         Immediately after the film-forming                              
          procedure, the resultant tubular film was                       
          easily directly opened.                                         
4         Immediately after the film-forming                              
          procedure, the resultant tubular film was                       
          easily opened by a light crumpling of the                       
          film by hand.                                                   
3         Immediately after the film-forming                              
          procedure, the resultant tubular film was                       
          opened by a strong crumpling of the film by                     
          hand.                                                           
2         Immediately after the film-forming                              
          procedure, the resultant tubular film was                       
          opened by a very strong crumpling of the                        
          film by hand.                                                   
1         Immediately after the film-forming                              
          procedure, the resultant tubular film was                       
          opened only by using an adhesive tape.                          
______________________________________                                    
 Note: When an inside metal surface of a filmforming machine is abraded by
 a polymer melt containing the antiblocking agent, the resultant polymer  
 film is colored by the abraded metal. The extent of the coloration of the
 polymer film can be confirmed by observing the end face of the film.     
 Accordingly, the color of the end face of the exemplified film was       
 observed by the naked eye, at an edge face of a roll formed of a wound   
 film.                                                                    
 
    
     The results of the above tests are shown in Table 1. 
     In Comparative Example 1, the same procedures as those described in Example 1 were carried out except that the antiblocking agent was not mixed into the polypropylene resin. The results of the tests are shown in Table 1. 
     In Comparative Example 2, the same procedures as in Example 1 were carried out except that the antiblocking agent of the present invention was replaced by 2,000 ppm of a conventional antiblocking agent consisting of silica powder and available under the trademark of Sailoid 244, made by Fuji Debison Co. 
     The results of the tests are shown in Table 1. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                                 Testing                                  
                                     Comparative                          
                                 method                                   
                                     Example                              
          Item                   ASTM                                     
                                     1   2    Example 1                   
__________________________________________________________________________
Antiblocking agent                                                        
          Type                   --  None                                 
                                         Silica                           
                                              Fibrous magnesium           
                                         particle                         
                                              oxysulfate particles        
          Amount            (ppm)                                         
                                 --  --  2,000                            
                                              2,000                       
Resultant Haze              (%)   D1003                                   
                                        1.2                               
                                            3.0                           
                                                 2.5                      
polypro-  Glossiness        (%)  D523                                     
                                     130 105  115                         
pylene    Static friction coefficient (*.sub.1)                           
                                  D1894                                   
                                       &gt;1.0                               
                                            0.69                          
                                                 0.56                     
film      Dynamic friction coefficient (*.sub.2)                          
                                  D1894                                   
                                       &gt;1.0                               
                                            0.68                          
                                                 0.52                     
          Tensile modulus of elasticity                                   
                            MD(*.sub.3)                                   
                                 D882                                     
                                     7,930                                
                                         7,480                            
                                              7,780                       
          (kg/cm.sup.2)     TD(*.sub.4)                                   
                                 D882                                     
                                     7,520                                
                                         7,450                            
                                              7,450                       
          Tensile yield point strength                                    
                            MD   D882                                     
                                     248 230  241                         
          (kg/cm.sup.2)     TD   D882                                     
                                     219 215  216                         
          Tensile break point strength                                    
                            MD   D882                                     
                                     792 742  790                         
          (kg/cm.sup.2)     TD   D882                                     
                                     748 693  732                         
          Ultimate elongation (%)                                         
                            MD   D882                                     
                                     764 748  778                         
                            TD   D882                                     
                                     821 830  800                         
          Antiblocking property of film to film                           
                                 --   1   4    5                          
          Color or end face of film                                       
                                 --  Light                                
                                         Light                            
                                              White                       
                                     black                                
                                         brown                            
__________________________________________________________________________
 Note:                                                                    
 (*.sub.1), (*.sub.2)  The inside surface portions of the tubular film wer
 rubbed against each other.                                               
 (*.sub.3)  MD: Longitudinal direction of the film                        
 (*.sub.4)  TD: Transverse direction of the film                          
 
    
     Table 1 clearly shows that the polypropylene film of Example 1 containing the fibrous magnesium oxysulfate particles as the antiblocking agent of the present invention exhibits a greater transparency, glossiness, lubricity, and antiblocking property than the comparative films of Comparative Examples 1 and 2. Also, since the end face of the polypropylene fiber of Example 1 had a white appearance, it was confirmed that the fibrous magnesium oxysulfate particles substantially do not abrade the inside metal surface of the film-forming machine. 
     Accordingly, it is clear that the fibrous magnesium oxysulfate particles are unexpectedly excellent for use as an antiblocking agent for plastic films. 
     EXAMPLE 2 AND COMPARATIVE EXAMPLE 3 
     In Example 2, a propylene homopolymer powder having a melt flow index of 2.0 g/10 min was mixed with 1,000 ppm of an antioxidant consisting of 2,6-di-t-butylhydroxytoluene (BHT) and 650 ppm of fibrous magnesium oxysulfate particles having a length of 15 μm, a thickness of 0.3 μm and a specific surface area of 8 m 2  /g, and the resultant mixture was melt-extruded at a temperature of 220° C., and converted to pellets, in the same manner as in Example 1. 
     The resultant pellets were converted to a sheet having a thickness of 1 mm, by a compression molding method at a molding temperature of 220° C. and a pressure of 100 kg/cm 2 , for a preheating time of 2 minutes and a heat-compressing time of 2 minutes, and the resultant sheet was rapid-cooled in a cooling water bath at a water temperature of 20° C. 
     The sheet was bi-axially drawn, using a bi-axial drawing machine, at a longitudinal draw ratio of 5 and a transverse draw ratio of 8, (at an area expansion ratio of 5×8=40) at a temperature of 140° C. 
     The resultant drawn film had a thickness of 20 μm and exhibited a satisfactory transparency and glossiness. 
     Ten pieces of the drawn film were superimposed on each other, and the resultant superimposed film pieces were pressed under a load of 36 g/cm 2  and left in a constant temperature oven at a temperature of 40° C. and a relative humidity (RH) of 80% for 40 hours. 
     After the above press treatment, the film pieces were easily peeled apart by hand. 
     In Comparative Example 3, the same procedures as mentioned in Example 2 were carried out except that an antiblocking agent was not mixed in the polypropylene film. The resultant comparative film had a satisfactory transparency and glossiness. After the above press treatment, the superimposed film pieces were in the form of a plate and could not be easily peeled apart by hand. 
     The results of Example 2 and Comparative Example 3 clearly indicate that the fibrous magnesium oxysulfate particles are very effective as an antiblocking agent for the plastic films. 
     EXAMPLE 3 
     The same procedures as in Example 1 were carried out with the following exception. 
     The polypropylene was replaced by a low density polyethylene having a melt flow index of 2.0 g/10 min, and the fibrous magnesium oxysulfate particles were mixed in an amount of 1,000 ppm in the polyethylene. 
     The polyethylene pellets were converted to a tubular film by an air-cooled inflation film-forming machine, at a resin temperature of 170° C., and the resultant tubular film had a thickness of 30 μm and a flattened tube width of 200 mm. 
     Immediately after the film-forming procedure, the resultant tubular film exhibited an excellent opening property and a very small haze of 5.5%. 
     EXAMPLE 4 
     The same procedures as in Example 1 were carried out, with the following exception. 
     The polypropylene was replaced by a copolymer of ethylene (90% by weight) with vinyl acetate (10% by weight) having a melt flow index of 0.3 g/10 min, and the fibrous magnesium oxysulfate particles were used in an amount of 1,200 ppm. 
     The air-cooling inflation film-forming procedure was carried out at a resin temperature of 130° C. and the resultant tubular film had a thickness of 30 μm and a flattened tube width of 200 mm. 
     The resultant tubular film exhibited an excellent opening property even immediately after the film-forming procedure, and a small haze of 9.0%. 
     COMPARATIVE EXAMPLE 4 
     The same procedures as described in Example 1 were carried out except that the fibrous magnesium oxysulfate particles were used in a large amount of 50,000 ppm. 
     The resultant film had an undesirable appearance due to the formation of gel particles, which were considered to consist of secondary agglomerates of the fibrous magnesium oxysulfate particles in the surface portion of the film, and thus was commercially useless.