The many applications for polyolefin film materials include their use as back sheet materials in the construction of diapers, bed pads and similar articles. Such articles are normally constructed of an absorbent inner layer disposed between a non-woven top sheet and a back sheet film. Films based on olefin polymers such as low density polyethylene, linear low density polyethylene, polypropylene and copolymers of ethylene with one or more comonomers such as vinyl acetate, acrylic acid, acrylic acid esters and other olefin monomers containing from 3 to 8 carbon atoms have been particularly preferred for such uses. These materials can be formulated to possess the requisite properties of flexibility and moisture impermeability which render them suitable for such applications.
It is also desirable that film materials used in such applications meet other criteria. For example, because such films come into contact with the user, they should be soft to the touch similar to cloth materials, and also, be quiet when rumpled or crinkled. They should at the same time also possess good physical properties such as tensile, elongation, puncture strength and impact strength so that they are not readily torn or punctured under conditions of normal or more rugged use.
One of the more preferred polymer materials which has been used in the preparation of the aforementioned film materials is linear low density polyethylene (LLDPE), mainly because of its film strength and toughness. LLDPE may be produced by copolymerizing ethylene with a C.sub.4 to C.sub.10 alpha-olefin. Generally the preferred alpha-olefins include those selected from butene-1, pentene-1, hexene-1, 4-methylpentene-1, heptene-1, and octene-1. The comonomers are present in amounts up to 20 weight percent, normally, between 3 and 14 weight percent(wt. %). The polymerization is conducted at low pressure using a chromium catalyst or Ziegler catalyst and may be carried out in the gas phase. The LLDPE produced by such methods has a density between 0.900 and 0.935 g/cm3 and a melt index (MI) between 0.1 and about 25.0 grams/10 min, more preferably from about 0.1 to 5.0 grams/10 min. Manufacturing processes for the production of LLDPE are known such as disclosed in U.S. Pat. Nos. 4,076,698 and 4,205,021.
Despite the good properties of films made using LLDPE, the melt processing properties of the polymer tend to restrict the process conditions under which cast films having a uniform thickness can be made. For example, a typical process for preparing an embossed film for use as diaper backsheet material is the slot-die cast extrusion process wherein the polymer is heated to a temperature generally in the profile range of about 350.degree. F. to about 550.degree. F. in an extruder and extruded through a slot die into the nip of a steel and rubber roll system. Where the film is to be embossed, the steel roll is engraved with a pattern of either protrusions (a male embossing roll) or depressions (a female embossing roll). The rubber roll serves as a backing roll into which the steel roll pattern is impressed during the formation of the film. Thus the roll system serves both to film-form and texture the polyolefin material. The speed of the rubber and steel rolls is maintained to permit continuous embossing of the film and subsequent take up of the film on a wind up roller after it exits the steel/rubber roller nip.
In terms of process efficiency, the film manufacturer is highly motivated to adjust processing conditions such as temperature, roll speeds and take-up speeds to maximize the rate of film production on the one hand while on the other hand producing a quality film having a substantially uniform film thickness, which is generally in the range of about 0.75 to 2 mils.
A particular property of LLDPE which tends to restrict the line speed at which film made from this polymer can be processed is caused by a phenomena referred to as draw resonance. Draw resonance is defined as the inability of the film to hold a constant film thickness down the film web in the machine or take-up direction, which is evidenced by the presence of light and dark bands running across the film. The light areas are a thinner gauge and the darker areas are of a thicker gauge. The degree of draw resonance is a direct function of line speed, becoming worse at higher line speeds. As a result, the maximum line speed at which good quality film can be produced is restricted by the onset of draw resonance.
Attempts have been made to improve the draw resonance of LLDPE by utilizing a blend of this material with up to about 50% by weight of another polyethylene having a higher Melt Index than LLDPE, such as high pressure low density polyethylene (LDPE) having a Melt Index of at least about 7.5. dg/min and a density in the range of from about 0.910 to 0.930 g/cm.sup.3 or as an alternative EVA's can be blended with LLDPE. However, draw resonance is still found to occur at higher line speeds (e.g., 500 ft/min) with such blends and the presence of a substantial amount of the LDPE in the blend serves to diminish the good tensile and elongation properties of film material when compared with material containing LLDPE as the sole polymer.
Recently, a new class of low density ethylene polymers have been introduced into the marketplace. These materials are characterized as very low density ethylene polymers having a density in the range of from about 0.88 to about 0.915 g/cm3, a melt index of from about 0.5 to about 7.5 dg/min, a molecular weight distribution (MWD) of no greater than 3.5 and a compositional distribution breadth index (CDBI) greater than about 70 percent. Compositionally, these polymers contain ethylene copolymerized with up to about 15 mole % of another alpha monoolefin containing from 3 to 20 carbon atoms such as butene-1, hexene-1, octene-1 and the like. These polymers with narrow MWD and narrow CD provide a unique balance of properties including narrow melting point range and improved toughness when fabricated into shaped articles as compared with normal low density polyethylene prepared by the conventional Ziegler catalyst systems.
One of the primary uses of such materials is as a heat seal component as is shown in copending U.S. appplication Ser. No. 916,736 filed Jul. 15, 1992 or layer used in conjunction with the preparation of packaging films constructed from polyolefin polymers such as polypropylene as is shown in copending U.S. application Ser. No. 660,402 filed Feb. 21, 1991 both of which are herein fully incorporated by reference. The narrow melting point range of these polymers renders them extremely useful in packing fill operations where it is desirable to develop strong seal strength immediately after packaging and while the film is still hot from the sealing operation.