Patent Publication Number: US-4579912-A

Title: Films of blends of linear ethylene polymers and aromatic polymers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of copending application Ser. No. 553,901 filed Nov. 21, 1983, now abandoned, and of an application Ser. No. 676,979 filed Nov. 30, 1984 in the names of Peter J. Canterino and Richard G. Shaw as a continuation-in-part of Ser. No. 553,901. 
    
    
     BACKGROUND OF THE INVENTION 
     Linear low and medium density polyethylene copolymers (LLDPE), and linear high density polyethylene are known materials which are widely commercially available. LLDPE is also described in U.S. Pat. No. 4,076,698 which is incorporated herein by reference. A commercially important use of LLDPE is in films which are conventionally prepared by blown film extrusion. Such films have generally good properties but often exhibit undesirably low stiffness for some uses and have marginal MD tear characteristics. In accordance with this invention, MD tear can be improved by blending LLDPE with aromatic polymers such as polystyrene or poly(para-methylstyrene). Alternatively, the stiffness of LLDPE can be improved without loss of MD tear. At the same time processing is improved, particularly in reduced extruder torque and in better bubble stability in the blown film. 
     SUMMARY OF THE INVENTION 
     In one aspect, this invention relates to films comprising blends of linear ethylene polymers and small amounts of, up to about 10 weight percent of aromatic polymers such as polystyrene or poly(para-methylstyrene). Another aspect of the invention relates to a method for reducing extruder torque by the use of such blends. Still another aspect of the invention relates to the method of improving the bubble stability in blown films made from such ethylene polymers by incorporating the aromatic polymer. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with this invention, the MD tear strength of ehtylene polymers, particularly LLDPE can be improved or, as is often desirable, the same MD tear strength can be obtained in a film of greater stiffness (see Secant modulus). This is accomplished by blending up to about 10 weight percent, preferably up to about 8 weight percent, and most preferably between about 3 and 8 weight percent of an aromatic polymer. 
     The ethylene polymers which are suitable for use in this invention are those prepared by low pressure techniques. Many such polymers are commercially available. Particularly suitable are LLDPE resins which are copolymers of ethylene and higher olefins, particularly 1-butene, 1-hexene, and 1-octene. These LLDPE copolymers generally have a density of between 0.90 and 0.94, preferably between 0.91 and 0.93. 
     Polystyrene and poly(para-methylstyrene) resins are particularly suitable as the aromatic polymer which is blended with the ethylene polymer. Preferably, the aromatic polymer is selected so that its melt viscosity is less than the melt viscosity of the ethylene resin used. The disparity in viscosity appears to contribute to better Elmendorf tear strength of the blends. Also, the best tear strength is obtained with blends in which the LLDPE itself has good tear properties. Such blends result in films with good tear and impact properties, and the higher modulus characteristic of a higher density LLDPE film. 
     Compatibilizers can be used in small amounts up to about 2 weight percent of the composition. Suitable compatibilizers include block copolymers of styrene-ethylene propylene-styrene (Kraton G), block copolymers of styrene and ethylene (Shelvis) and ethylene propylene copolymers (Vistalon 3707). The resin blends and films are made in the conventional manner. Preferably, films are made by blown film extrusion using conventional techniques to obtain films which are generally from about 0.5 to 5 mils in thickness. 
    
    
     The following non-limiting examples are illustrative of this invention. Unless otherwise noted all parts are by weight. 
     EXAMPLES 1-10 
     Blends of various linear low density copolymers (LLDPE) with paramethylstyrene polymers (PPMS) were compounded at 160° C. with a Brabender twin screw extruder. Films having a 1.5 mil gauge were produced at 2:1 BUR, 40 mil gap at about 18 g/min output. Film properties are summarized in Table I below. 
     
                       TABLE I                                                     
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                Film Properties                                           
                 Weight                 Spencer                           
                 Percent  MD Tear                                         
                                 TD Tear                                  
                                        Impact                            
Ex.   LLDPE      PPMS     (g/mil)                                         
                                 (g/mil)                                  
                                        (g/mil)                           
______________________________________                                    
--    Dowlex 2045                                                         
                 0        412    733    1057                              
1     Dowlex 2045                                                         
                 5        500    612    903                               
2     Dowlex 2045                                                         
                 10       306    444    356                               
--    Dowlex 2049                                                         
                 0        261    672    529                               
3     Dowlex 2049                                                         
                 5        393    490    358                               
4     Dowlex 2049                                                         
                 10       237    399    252                               
--    Dowlex 2047                                                         
                 0        402    683    478                               
5     Dowlex 2047                                                         
                 5        493    511    382                               
6     Dowlex 2047                                                         
                 10       488    554    255                               
--    GPX-1      0        144    325    551                               
7     GPX-1      5        145    259    221                               
8     GPX-1      10       109    166    204                               
--    GP-2       0        128    325    311                               
9     GP-2       5        170    251    192                               
10    GP-2       10       168    207    150                               
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     The Dowlex LLDPE resins are ethylene-octene copolymers commercially available from Dow Chemical Company. 
     GPX-1 and GP-2 are ethylene-butene copolymers having a density of about 0.918 and Melt Indexes of about 1 and 2 respectively. 
     The PPMS is a free-radical catalyzed resin having a Mw of about 300,000 and a Mw/Mn of about 3.5. 
     EXAMPLE 11 
     Substitution of polystyrene for the PPMS in Example 10, in amounts of 8 to 10 weight percent, gives a blend with enough elasticity to give a stable bubble at higher output rates than the LLDPE alone. Films made of the blend exhibit similar or improved MD tear compared to LLDPE alone but with a desirable increase in stiffness (Secant Modulus). 
     EXAMPLE 12 
     Blends of ethylene-hexene copolymer and polystyrene gave films with improved stiffness and acceptable MD tear strength for that stiffness. Suitable ethylene-hexene copolymers include a copolymer having a Melt Index of 0.75 and a density of 0.927 and a copolymer having a Melt Index of 1.0 and a density of 0.92. 
     Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be restored to, without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims.