BACKGROUND OF THE INVENTION Metallocene-prepared polyolefin films, especially polyethylene films, are frequently formed as blown films. Film blowing involves continuous- ly extruding a polymer melt through a die in order to form a continuous cylinder of viscous polymer, and then expanding the diameter of the cylinder through, for example, a pressure differential between the inside and outside of the cylinders.

When the cylinder is at a desired radius which is approximately equal to n times the reciprocal of the width of a take-up spool, and when the cylinder's wall thickness is approximately equal to the desired film thickness, the film is parted longitudinally and then wound onto the take-up spool.

Metallocene-prepared polyolefin films generally provide significant improvements in physical properties over their Ziegler-Natta-prepared counterparts. However, one property in which further improvement is thought to be possible is"tear strength". Film tearing can occur longitudinally along the film (also known as the"machine direction "or"MD") or transversely across the film (known as the"transverse direction"or"TD"). Tear strength can be increased by increasing the blown-film thickness ; however, this approach is costly and can result in less productive processing conditions.

There is therefore a need for a method for reducing tearing and increasing tear strength in blown polyolefin films.

SUMMARY OF THE INVENTION In one embodiment, the invention is a method for reducing tear strength in a blown film formed from a metallocene-prepared polyethylene melt.

The method comprising adding to the melt an effective amount of a nucleating agent selected from the group consisting of dibenzylidene sorbitol, paramethyl dibenzylidene sorbitol, dimethyl dibenzylidene sorbitol, ethylene copolymers containing grafted maleic anhydride, ethylene copolymers containing grafted acrylic acid, and mixtures thereof.

In another embodiment, the invention is a composition comprising a metallocene-prepared polyethylene melt and a nucleating agent selected from the group consisting of dibenzylidene sorbitol paramethyl dibenzylidene sorbitol, dimethyl dibenzylidene sorbitol, ethylene copolymers containing grafted maleic anhydride, ethylene copolymers containing grafted acrylic acid, and mixtures thereof.

DETAILED DESCRIPTION OF THE INVENTION Film tearing can be reduced in a process for forming a blown metallocene-prepared polyolefin film from a melt by adding a nucleating agent to the melt. The preferred nucleating agents are sorbitol-based materials, phthalocyamine-based pigments, acrylic acid or maleic anhydride ethylene copolymers. The sorbitols are selected from dibenzylidene sorbitol, paramethyl dibenzylidene sorbitol, or dimethyl dibenzylidene sorbitol. Paramethyl dibenzylidene sorbitol is particularly preferred. Mixtures of the aforementioned nucleating agents may also be used. The additive may also be used together with sodium stearate as an acid-neutralizing agent present in a concentration ranging from about 100 to about 1,000 parts per million.

Polyethylene useful in the practice of the invention ranges in melt index from about 0.5 dg/min to about 3.0 dg/min with about 0.5 dg/min to about 1.5 dg/min being preferred. Density ranges from 0.915 g/cm3 to about 0.925 g/cm3, with about 0.917 g/cm3 to about 0.920 g/cm3 being preferred.

Melts formed from the polyethylene and the nucleating agents of the invention are compatible with conventional film-blowing equipment. Methods of combining the polyethylene and nucleating agent include melt-homogenization and tumbling powdered polyethylene resin and powdered nucleating agent together in a barrel blender.

The graft copolymers used in this invention range in density from <BR> <BR> <BR> about 0.92 g/cm3 to about 0.96 g/cm3, with. 95 g/cm3 being preferred in the case<BR> <BR> <BR> <BR> <BR> of ethylene-acrylic acid and. 92 g/cm3 preferred in the case of maleic anhydride.

In the case of ethylene-maleic anhydride graft copolymers, the copolymer may also contain about 1 to about 25 butene branches per thousand carbon atoms, with about 10 to about 18 branches being preferred. The acrylic acid level in the ethylene acrylic acid graft copolymers is about 2 to about 8% with about 5 to about 6% being preferred. The maleic anhydride level in the ethylene maleic anhydride graft copolymer is about 0.25 to about 2%, with 1% being preferred.

Copolymer nucleating agents useful in the invention range in melt index from about 0.5 dg/min to about 10 dg/min with about 1 dg/min to about 6 dg/min being preferred.

TABLE 1 Sample 1 2 3 4 5 6 0.5 wt.% 0.1 wt.% 0.5 wt.% Paramethyl 0. 25% dimethyl dimethyl 5. 0 wt. % Nucleating - dibenzylidene ethylene-acrylic dibenzylidene dibenzylidene ethylene-acrylic Agent #1 sorbitol acid copolymer sorbitol sorbitol acid copolymer Nucleating 0.1 wt.% 0.1 wt.% Agent#2 - sodium stearate - sodium stearate - - I2 (dz/mm) 1.05 1.05 1.05 1.05 1.05 1.05 I21 (dg/mm) 15.5 15.5 15. 5 15. 5 15.5 15.5 Resin Density~0. 9190 0. 9190 0. 9190 0.9190 0.9190 0.9190 Melt Temperature °C 434 438 439 442 439 432 TABLE 2 SAMPLE 1 23456 Gauge~~~~~~mil 1.057 1.022 1.157 1.104 1.108 1.017 Gauge mil 0.048 0.062 0.032 0.070 0.080 0.052 Variabili 1% Secant MD psi 28040 34600 25720 28100 27520 30520 Modulus TD psi 26700 34770 30600 26370 30750 32270 Elmendorf MD g 266 334 322 316 351 290 Tear-initial~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~ MD mil 270 334 292 299 301 292 30 daysMDg266 359 290 326 342 256 MD mil 259 345 261 310 303 253 Elmendorf TD g 462 572 503 457 514 420 Tear-initial~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~ TD/mil 440 564 452 415 447 444 30 daysTDg422 567 458 458 514 448 TD mil 405 556 415 415 476 451 The nucleating agents of the present invention provide enhanced properties in blown polyethylene films. An effective amount of the nucleating agent, or mixtures thereof, is added to a polyethylene melt prior to film processing. The effective amount generally ranges from about 0.1 wt. % to about 5 wt. % based on the added weight of the polyethylene and nucleating agent, with the range of about 0.1 wt. % to about 0.5 wt. % being preferred for the sorbitol based nucleating agents, about 2.5 to about 5.0 wt. % is preferred for the graft copolymer nucleating agents.

Table 1 summarizes the physical properties of seven representative metallocene-prepared polyethylene melts that contain nucleating agents of the present invention. Such melts are compatible with conventional film-blowing equipment. The formulations may be melt homogenized or, alternatively, powdered polyethylene resin and powdered nucleating agents may be tumbled together in a barrel blender.

Table 2 summarizes physical properties of film blown from the melts of Table 1. Sample 1 is formed from base resin without additives.

Samples 2 through 6 all show improvement in machine direction and/or trans- verse direction tear strength both initially and 30 days after film formation. The improvement is obtained without a reduction in stiffness, and in two cases the modulus actually increases in the presence of the nucleating agent. Transverse direction tear strength showed an improvement over the control samples after 30 days in all cases and an initial improvement in all cases except Sample 4.

While not wishing to be bound by any theory, it is believed that a metallocene-prepared polyolefin film's tear strength is related to the material's crystallization time. Generally, polyolefin films wherein the polymer crystallizes in the form of oriented lamellae exhibit short crystallization times. Films crystallizing isotropically typically exhibit longer crystallization times and lower tear strength. Crystallization time and its measurement are well-known to those skilled in the art. Crystallization time can be measured, for example, by recording the width of a crystallization exotherm.

Accordingly, in the practice of the present invention, a nucleating agent is added to the melt in order to decrease crystallization time and consequently form a heterogeneous polymer film of high tear strength.

It is believed that nucleating agents capable of increasing tear strength in metallocene-prepared polyethylene films are polymers whose chains can elongate in the machine-direction of the blown film, or alternatively, commercial nucleating agents that can form linear regions of physical gelation oriented in the machine direction of the blown film. According to this model, such nucleating agents provide sites along their backbone for nucleation of the favored lamellae structure during solidification. Lamellae nucleate perpendicular to these elongated or oriented regions, and it is believed that this morphology provides optimum tear strength.

Accordingly, sorbitol based nucleating agents are effective tear- strength improvers because they naturally form physical gels in the melt.

Ethylene-based nucleating agents of a higher density than the parent polymer exhibit tear strength improvement as well.

It should be noted that improvements in film optical properties such as haze and gloss also occur when the polyethylene melt contains a nucleating agent. However, polymer chain orientation and physical gelation are not believed to play a role in optical property improvements.