BACKGROUND OF THE INVENTION Polyolefin films, especially polyethylene films, are frequently formed as blown films. Film blowing involves continuously 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 approximately equal to n times the reciprocal of the width of a take-up spool and 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.

Increased transparency can be obtained in polypropylene films by adding nucleating agents. See, for example, U. S. Patent 4,611, 024. It is believed that spherulite formation in polypropylene affects film transparency. <BR> <BR> <BR> <BR> <P>Opacity (i. e., a loss in transparency) is believed to result from the presence in the film of spherulites larger than about 1 micron because such spherulites would scatter visible light. Adding and dispersing nucleating agents into the melt from which the film is blown increases transparency because average spherulite size is reduced to below 1 micron. Visible light is therefore not strongly scattered by the film.

Blown films prepared from conventional Ziegler-Natta catalyzed polyethylene can also suffer from reduced transparency. However, the opacity in Ziegler-Natta catalyzed polyethylene is believed to result, in part, from a different mechanism than that which is believed to cause opacity in polypropylene films. In Ziegler-Natta catalyzed polyethylene the opacity is believed to result in part from the presence of large spherulites, as in polypropylene, and in part from the presence of low molecular weight species such as waxes that naturally form during polymerization.

Metallocene-prepared polyolefin films generally provide significant improvements in physical properties, such as transparency, over their Ziegler-Natta-prepared counterparts. However, further transparency improvement is desirable.

There is therefore a need for nucleating agents that are effective methods for increasing transparency in blown polyethylene films.

SUMMARY OF THE PRESENT INVENTION In one embodiment, the invention is a method for providing a polyethylene melt from which films of increased transparency can be formed, the method comprising forming a melt of polyethylene and a nucleating agent selected from the group consisting of ethylene copolymers containing grafted maleic anhydride, ethylene copolymers containing grafted acrylic acid, and mixtures thereof.

In another embodiment, the invention is a composition comprising polyethylene and a nucleating agent selected from the group consisting of ethylene copolymers containing grafted maleic anhydride, ethylene copolymers containing grafted acrylic acid, and mixtures thereof.

DETAILED DESCRIPTION OF THE INVENTION Transparency can be increased in polyethylene films blown from a melt by forming the melt from a nucleating agent and polyethylene. The preferred nucleating agents are ethylene copolymers containing grafted maleic anhydride, ethylene copolymers containing grafted acrylic acid, and mixtures thereof, with acrylic acid-ethylene copolymers being particularly preferred. The additive may also be used together with sodium stearate as an acid neutralizing agent present in a concentration ranging from about 0.005 wt. % to about 0.1 wt. % based on the weight of the polyethylene and nucleating agent. The preferred concentration of sodium stearate, when present, ranges from about 0.01 wt. % to about 0.1 wt. % based on the weight of the copolymer and nucleating agent.

The nucleating agents of the present invention provide enhanced properties in blown polyethylene films, especially films blown from polyethylene formed in a process using at least one metallocene catalyst or co- catalyst. An effective amount of nucleating agent, or mixtures thereof, is added to or combined with 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.

Polyethylene useful in the practice of the invention ranges melt index from about 0.3 dg/min to about 4 dg/min. 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 as nucleating agents in this invention range in density from about 0.92 to about 0.96 g/cm3, with about. 95 g/cm3 being preferred in the case of ethylene-acrylic acid and about. 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 about 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 summarizes the optical properties of representative metallocene-prepared polyethylene melts that contain nucleating agents of the present invention. Films were blown from five different melts. The melts are labeled samples 1 through 5, respectively. Sample 1 contained Ziegler-Natta polymerized polyethylene and none of the nucleating agents of the present invention. Sample 2 contained. 25 wt. % ethylene-acrylic acid copolymer with the balance being Ziegler-Natta polymerized polyethylene. Sample 3 contained polyethylene prepared in the presence of a metallocene catalyst and none of the nucleating agents of the present invention. Sample 4 contained polyethylene prepared in the presence of a metallocene catalyst and 0.25 wt. % ethylene- acrylic acid copolymer. Sample 5 contained polyethylene prepared in the presence of a metallocene catalyst and 0.25 wt. % ethylene-butene copolymer containing maleic anhydride grafts. Polyethylene films were blown from each melt, and haze values for each sample are also recorded in the Table. Haze is a measure of opacity; higher haze values indicates lower transparency.

0. 001 inch thick films were extruded at a rate of about 150 lineal feet per minute. Melt temperature ranged from about 400 to about 450°C.

TABLE 1 POLYMER SAMPLE 1 SAMPLE 2 SAMPLE 3 SAMPLE 4 SAMPLE S - Measured Haze 20% 9.20% 9.90% 3.70% 5% The Table shows that a factor of two improvements in film transparency is obtained when the film is formed from a melt in which the nucleating agents of the present invention are added to Ziegler-Natta polymerized polyethylene. This improvement is believed to result from the nucleating agent inhibiting spherulite formation during film blowing.

The Table also shows that metallocene-prepared polyethylene forms a film without nucleating agent that is as transparent as one formed from Ziegler-Natta polyethylene that contains nucleating agent. While not wishing to be bound, it is believed that the presence of the low molecular weight species such as waxes in the Ziegler-Natta polyethylene limits the ultimate transparency that can be obtained even when spherulite formation is suppressed.

Metallocene-prepared polyethylene has a much narrower molecular weight distribution, and the low molecular weight species are believed to be absent or present in very small concentrations.

When the nucleating agents of the present invention are added to metallocene-prepared polyethylene, as in Samples 4 and 5, still further reductions in haze are obtained compared to the Ziegler-Natta-prepared polyethylene films formed with or without nucleating agent. It is believed that this additional improvement is due to the inhibition of spherulite formation, films formed from the metallocene-prepared polyethylene being already substantially free of low molecular weight species.

In summary, it has been discovered that combining a nucleating agent with a polyethylene melt when forming blown films formed from either metallocene-prepared or Ziegler-Natta-prepared polyethylene causes a substantial increase in film transparency.

It is believed that the opacity in metallocene-prepared polyethylene films results from the presence of large spherulites in the films. Spherulite growth is inhibited when nucleating agents are present in the melt from which the film is blown. Although spherulite growth is somewhat suppressed in <BR> <BR> <BR> <BR> conventionally-prepared (i. e., Ziegler-Natta-prepared) polyethylene because the high molecular weight species present in that material function as a nucleating agent, adding the nucleating agents of the present invention to conventionally- prepared polyethylene melts results in greater spherulite growth suppression, and films blown therefrom exhibit increased transparency.

Preparing the polyethylene in a metallocene polymerization process results in the absence of such high molecular weight material, leading consequentially to large spherulite formation in the absence of an added nucleating agent. However, adding nucleating agents of the present invention to the melt results in blown films having the greatest transparency because, it is believed, low molecular weight species that partially cause increased opacity in conventionally-prepared polyethylene films are substantially absent in the metallocene-prepared materials.