LLDPE composite film free of melt fracture

In accordance with the invention a linear low density polyethylene is coextruded with a composition containing a LDPE and/or HDPE. The product film exhibits substantially no melt fracture and no surface defects.

FIELD OF THE INVENTION 
This invention relates to a composite film comprising a layer of linear low 
density copolymer of ethylene and a minor amount of an olefin containing 4 
to 10 carbon atoms (LLDPE), and to films of improved properties formed 
from such compositions. Also included in the invention is the process of 
forming films from such compositions. The resulting films are 
characterized by at least substantial retention of the desirable 
properties of the linear low density polyethylene and the substantial 
elimination of melt fracture of the film. 
BACKGROUND OF THE INVENTION 
Linear low density polyethylenes exhibit extremely desirable properties in 
film products. However, processing characteristics of the linear low 
density polyethylene (LLDPE) are not equal to the inherent properties of 
the resin itself. Commercial application of LLDPE resins is limited partly 
because of extrusion problems, including high back pressure and power 
consumption due to extremely high shear viscosity, and partly because of 
film surface defects due to melt fracture. 
Accordingly, processing aids have been employed in conjunction with the 
linear low density polyethylene resins to improve those processing 
characteristics. For example, the extrudability of film grade LLDPE with 
narrow molecular weight distribution can be improved by blending. Binary 
blends containing high amounts (10 to 15%) of polypropylene or polystyrene 
with the LLDPE exhibit improved processing characteristics, but the high 
levels (10 to 15%) of polypropylene or polystyrene reduce the properties 
of the base LLDPE so greatly that little or no property advantages are 
seen in using the LLDPE. 
Extrusion of these binary blends, even containing lower amounts of either 
polypropylene or polystyrene, results uniformly in products of 
unacceptable appearance. Particularly, these products consistently exhibit 
melt fracture and a surface of irregularities which are striated or wavy 
in configuration. Most attempts at remedies of melt fracture are physical 
or mechanical in nature. For example, it has been proposed to vary die 
geometry and increasing dies gap. These are solutions which invoke other 
problems. Modification of die geometry is costly and increasing dies gap 
or processing temperature create the problem of bubble instability for the 
blown film processing. 
SUMMARY OF THE INVENTION 
In accordance with the invention a linear low density polyethylene, or a 
blend thereof, is coextruded with a compostion containing a LDPE and/or 
HDPE. The product exhibits good processability with substantial retention 
of inherent properties of the LLDPE. 
In accordance with the invention a linear low density polyethylene is 
coextruded with a composition containing a LDPE and/or HDPE. The product 
film exhibits substantially no melt fracture and no surface defects. 
In accordance with the invention a linear low density polyethylene is 
coextruded with a LDPE and/or HDPE, to produce a product which exhibits 
good processability with substantial retention of inherent properties of 
the LLDPE and which is free of melt fracture and substantially free of 
surface defects.

DETAILED DESCRIPTION OF THE INVENTION 
The composite film of the invention comprises two lamina. One of the two 
lamina comprises LLDPE and/or a LLDPE blend, which constitutes a 
substantial portion of the composite, as a base layer. The second lamina 
comprises HDPE, LDPE or admixtures thereof. Coextrusion of the 
compositions of these two lamina results in a LLDPE film free of melt 
fracture. Accordingly, the composite of the invention can be said to 
consist essentially of these two laminae. 
In accordance with the invention a substantial LLDPE component and a lesser 
quantity of a composition containing polystyrene is coextruded with a 
polymer selected from the group consisting of LDPE, HDPE and admixtures 
thereof. The product is substantially free of melt fracture and free of 
surface defects. 
The base layer can comprise linear low density polyethylene or linear low 
density polyethylene blended with polystyrene. Linear low density ethylene 
copolymers (LLDPE) of this invention are linear copolymers of ethylene and 
a minor amount, preferably about 2 to 10 wt. %, of an olefin, preferably a 
1-olefin, containing 4 to 10 carbon atoms and having a density of about 
0.9 to 0.94, a melting point of about 110.degree. to 130.degree. C., and a 
melt index of about 0.2 to 10. The preferred olefin comonomers are 
1-butene, 1-hexene and 1-octene. The LLDPE may be prepared by any of 
various methods known in the art, e.g., by relatively low pressure methods 
as disclosed, for example, in U.S. Pat. No. 4,076,698, or by relatively 
high pressure methods, as disclosed, for example, in U.S. Pat. Nos. 
4,405,774 and 4,530,983. LLDPE exhibits a density of less than 0.94 
g/cm.sup.3. 
The LLDPE blends which are coextruded in accordance with the invention can 
contain up to 15%, generally up to to 10% and most preferably 0.5 to 5% of 
polystyrene (percentage based on the blend.) Preferably, the polystyrene 
is a polystyrene homopolymer. However, poly(para-methylstyrene) resins may 
be employed as partial or complete substitutes for the polystyrene. 
Moreover, the aromatic polymer can also contain comonomers providing that 
the aromatic monomer is the predominat constituent. For instance, high 
impact polystyrene can be used. 
The LLDPE base layer comprises more than 50% of the composite, preferably 
more than 70% of the composite, most preferably more than 75% of the 
composite, 
The second layer coextruded with the LLDPE containing HDPE, LDPE or 
admixtures thereof, comprises less than 30 weight percent of the 
composite. In Examples below the second layer comprises up to 25% of the 
composite. In Examples below the second layer cancomprises up to 20% of 
the composite, and even up to to 10% of the composite. 
The LDPE and HDPE compositions are well known in the art and are 
commercially available. LDPE, an acronym for low density polyethylene, its 
physical and chemical properties and its productsion, is described in KIRK 
OTHMER, Encyclopedia of Chemical Technology, Third Edition, Vol. 16, pages 
402-420 (1981). LDPE is generally produced by high pressure methods, 
compared to those used for LLDPE production HDPE, and acronym for high 
density polyethylene, is described in KIRK OTHMER, Encyclopedia of 
Chemical Technology, Third Edition, Vol. 16, pages 421-433 (1981). HDPE 
has a density of at least 0.94 g/cm.sup.3. 
The lamina or blends used in the invention can contain commercial 
quantities of color and antiblocking agents. In addition to the polymer 
components, the blend may also contain any of various additives 
conventionally added to polymer compositions for different purposes, such 
as microtalc, stabilizers, compatibilizers, pigments, etc. 
The blends used in the invention may be prepared using any of various 
methods known in the art. For example, pellets of the two polymers may be 
dry blended and the blend added directly to a blown film extruder, e.g., a 
Sterling extruder, to produce films having a thickness, for example, of 
about 0.5 to 5 mils. Blown film is relatively thin as compared to film or 
sheet used in molding operations and is generally used in flexible 
packaging. In this process a narrow molecular weight distribution polymer 
is melt extruded through an extruder. Tubular products, for example, bags, 
can be produced. The tube may also be slit to form flat film. The film may 
also be oriented. 
The following examples further illustrate the invention. 
EXAMPLES 
Films were made in a 21/2 inch Sterling film extruder with a 6 inch annular 
die with two layer coextrusion via two 21/2 extruders. 
The LLDPE used in these examples was hexene copolymer of polyethylene from 
Mobil (NXT-081, MI=0.65 and density=0.922 g/cm.sup.3.) The polystyrene 
(PS) was from Mobil (PS 2120). The high density polyethylene (HDPE) was 
from Mobil (HTA -101) and from Cain (L5005). The low density polyethylene 
(LDPE) was from Mobil (LCA 113 and LBA 133). 
The results are set forth in the following Table: 
______________________________________ 
Melt Facture and Surface Texture of LLDPE Films 
Layer Melt Surface Surface 
Resin Ratio Fracture Appearance 
Texture 
______________________________________ 
LLDPE + PS/ 
90/10 Severe Wavy Same 
LLDPE + PS 
LLDPE + PS/ 
80/20 Severe Wavy Same 
LLDPE + PS 
LLDPE + PS/ 
90/10 None Smooth Differential 
HDPE 
LLDPE + PS/ 
80/20 None Smooth Differential 
HDPE 
LLDPE + PS/ 
80/20 None Smooth Differential 
HDPE + LDPE 
LLDPE + PS/ 
90/10 Slight Smooth Differential 
LDPE 
LLDPE + PS/ 
75/25 None Smooth Differential 
LDPE 
LLDPE + PS = 96.5/3.5 of LLDPE/PS 
HDPE + LDPE = 90/10 of HDPE/LDPE 
By way of explanation it is noted that LLDPE was contained in a blend of 
LLDPE and PS (polystyrene.) 
Thus it is apparent that there has been provided, in accordance with the 
invention, a polymeric composite that fully satisfies the objects, aims, 
and advantages set forth above. While the invention has been described in 
conjunction with specific embodiments thereof, it is evident that many 
alternatives, modifications, and variations will be apparent to those 
skilled in the art in light of the foregoing description. Accordingly, it 
is intended to embrace all such alternatives, modifications, and 
variations as fall within the spirit and broad scope of the appended 
claims.