Abstract:
Linear low density copolymers (LLDPE) are blended with up to about 10 weight percent of an aromatic polymer, e.g., polystyrene or poly(para-methylstyrene), to give films having improved MD tear strength or equivalent MD tear strength and higher stiffness compared to the LLDPE alone. Also, extruder torque is lower and bubble stability is greater in blown film extrusion with the blends as well as in blends in which medium or high density polyethylene is used instead of the LLDPE.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of Ser. No. 771,965 filed Sept. 3, 1985, and a continuation-in-part of Ser. No. 685,583 filed Dec. 24, 1984, now U.S. Pat. No. 4,579,912, and a continuation-in-part of application Ser. No. 676,979 filed Nov. 30, 1984, now abandoned in the names of Peter J. Canterino and Richard G. Shaw and a contintuation-in-part of Ser. No. 553,901 filed Nov. 21, 1983, now abandoned. 
    
    
     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 and splittiness characteristics. In accordance with this invention, MD tear and splittiness can be improved by blending LLDPE with aromatic polymers such as polystyrene or poly(para-methylstyrene). Alternatively, the stiffness of LLDPE can be improved with little or no loss of MD tear strength. 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 such as high density polyethylene and LLDPE 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 ethylene 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 linear ethylene polymers which are suitable for use in this invention can be 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______________________________________          Film Properties           Weight                  Spencer           Percent  MD Tear                           TD Tear ImpactEx.  LLDPE      PPMS     (g/mil)                           (g/mil) (g/mil)______________________________________--   Dowlex 2045           0        412    733     10571    Dowlex 2045           5        500    612     9032    Dowlex 2045           10       306    444     356--   Dowlex 2049           0        261    672     5293    Dowlex 2049           5        393    490     3584    Dowlex 2049           10       237    399     252--   Dowlex 2047           0        402    683     4785    Dowlex 2047           5        493    511     3826    Dowlex 2047           10       488    554     255--   GPX-1      0        144    325     5517    GPX-1      5        145    259     2218    GPX-1      10       109    166     204--   GP-2.sup.  0        128    325     3119    GP-2.sup.  5        170    251     19210   GP-2.sup.  10       168    207     150______________________________________ 
    
     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. 
     EXAMPLES 13-14 
     A compounded blend of 95% LLDPE (ethylene-1-hexene copolymer, MI 0.7, density 0.922) and 5% polystyrene (Mobil PS 1240) was produced by feeding both materials into an intensive mixer (Bolling Mill) at the correct ratio, melting and mixing both polymers together such that an intimate mixture was obtained, then feeding the molten mass to a pelletizer, thus producing pellets. These pellets were cooled and stored several weeks. The pellets were subsequently made into film suitable for producing grocery sacks by the blown film process. Similarly produced pellets of the LLDPE without polystyrene were also made into film in an identical manner. 
     In a second run, pellets of the identical polystyrene were blended with pellets of the LLDPE in the blown film extruder hopper in the proper ratio and immediately made into film suitable for grocery sacks. Films from both experiments were tested for 1% secant modulus (stiffness) and for tensile impact (splittiness). 
     
         ______________________________________                       MD 15    TD TensileExam-              Blend    Secant   Impactple   Film         Method   Modulus (psi)                                (ft-lb)______________________________________C-1.  100% LLDPE   Melt     37000    0.461.    95% LLDPE/5% Melt     48500    0.52 PS1240 % increase (1) vs (C-1)                   31     15C.2.  100% LLDPE   Dry      46200    0.272.    95% LLDPE/5% Dry      55000    0.47 PS1240 % increase (2) vs (C-2)                   19     76______________________________________ 
    
     The bags made in accordance with this invention are demonstrably easier to open than the corresponding bags containing no aromatic polymer. The ease of opening has been associated at least in part to a reduced tendency to block in standard quantitative blocking tests. The increased stiffness of the blends is also believed to contribute to the ease of opening. 
     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.