Oriented HDPE films with skin layers

Readily castable high density polyethylene (HDPE) films, preferably containing broad molecular weight distribution HDPE resin having treated upper skin layers and untreated lower skin layers, preferably coextruded, on each side. The treated upper skin layer is derived from a polymer, e.g., ethylene-propylene-butene-1 terpolymer, and the lower skin layer is derived from a polymer such as ethylene-propylene-butene-1 terpolymer and LDPE compounded with silicone oil. Such skin layers provide a film which is readily castable while maintaining the desirable properties of HDPE including dead fold, twist retention, TD tear and moisture barrier. The films are cast, and oriented in the solid state up to about two times in the machine direction and six times or more in the transverse direction to give films having good dead-fold characteristics making them highly suited for packaging, particularly for dry foods.

FIELD OF THE INVENTION 
This invention relates to thermoplastic films and more particularly to 
films of oriented high density polyethylene (HDPE) which have good 
dead-fold characteristics, twistability, printability and receptivity of 
water-based coatings. 
BACKGROUND OF THE INVENTION 
Blown films of HDPE having an ethylene-vinyl acetate heat seal coating are 
used for food packaging but such films must have a thickness of about two 
mils to meet the water vapor transmission (WVTR) requirements for 
packaging suitable for dry foods such as cereals. Moreover, blown HDPE 
films do not exhibit the dead-fold properties desirable in food packages 
particularly of the bag-in-box type. 
U.S. Pat. Nos. 4,870,122 and 4,916,025 (Lu) describe HDPE films which have 
good WVTR properties at film thicknesses of about one mil. The films are 
produced from high density polyethylene (HDPE) having a density of 0.96 or 
higher and are biaxially oriented in an imbalanced manner to a degree of 
about 1.25:1 to about 2:1 in the machine direction (MD) and to a degree of 
about 6:1 to about 12:1 in the transverse direction (TD). These films have 
reduced water vapor transmission (WVTR), improved dead-fold 
characteristics and other physical characteristics which are markedly 
better than blown HDPE films which can be further improved by the addition 
of microcrystalline wax. When provided with a heat seal layer by 
co-extrusion or coating, the films are particularly well suited for use in 
packaging foods in bag-in-box operations conducted on vertical, form, fill 
and seal (VFFS) machinery. While the imbalanced orientation confers the 
desired overall combination of properties on the film, the high degree of 
orientation in the transverse direction tends to make the film weak in 
this direction so that it tears relatively easily. 
U.S. Pat. Nos. 4,343,852, 4,400,428, 4,419,411, 4,502,263, 4,692,379, and 
4,734,317, all of which are incorporated herein by reference, disclose 
films having a base layer of polypropylene polymers and sealable skin 
layers on one or more sides of the base layer. However, these structures 
do not comprise HDPE base layer and typically lack good deadfold 
properties while having higher water vapor transmission rates associated 
with polypropylene. Given the differences between polypropylene and HDPE, 
e.g., in melting point (348.degree. F. for PP, 279.degree. F. for HDPE), 
processing polypropylene films requires different extrusion temperature, 
extruder screw design, die design draw down ratio, machine direction 
orientation temperature and percent, as well as transverse direction 
orientation temperature and rate. 
SUMMARY OF THE INVENTION 
The present invention relates to providing a film having good dead fold 
properties, optical clarity, good slip properties, and good receptivity to 
water based coatings, including water based inks, water based adhesives 
and other common water based coatings such as polyvinylidene chloride 
(PVDC), and acrylics. 
The present invention relates to a film of high density polyethylene (HDPE) 
that has coextruded skin resins, laminated film or coatings on both sides 
of the HDPE base layer. Up to three layers of skin resins can be on each 
side of the base layer and can include heat sealable or non heat sealable 
type skins. For present purposes heat sealable skin resins are those which 
seal at a minimum seal temperature below the distortion temperature of 
HDPE base layer film. 
More particularly, the present invention relates to an oriented polymer 
film which comprises: 
a) a treated upper skin layer (a) capable of receiving a water base 
coating, said layer (a) having a surface coextensively adherent to the 
upper surface of a core layer (b), and an exposed surface, said upper 
layer (a) being formed from at least one polymer selected from the group 
consisting of ethylene-propylene-butene-1 terpolymer, ethylene-propylene 
random copolymer, ethylene-propylene block copolymer, isotactic 
polypropylene, low density polyethylene (LDPE), very low density 
polyethylene (VLDPE), linear low density polyethylene (LLDPE), medium 
density polyethylene (MDPE), polyvinylidene chloride (PVDC), polyvinyl 
alcohol (PVOH) and acrylic, said polymer being compounded with an 
effective amount of anti-blocking agent, but being substantially devoid of 
silicone oil, the exposed surface of upper layer (a) containing a 
coefficient of friction-reducing amount of silicone oil transferred to 
said surface of (a) by contact with a silicone oil present upon the 
exposed surface of a lower skin layer (c); 
b) a base layer (b) derived from HDPE, having an upper surface and lower 
surface; 
c) a lower skin layer (c) having a surface adherent to said lower surface 
of base layer (b), and an exposed surface, said layer (c) containing at 
least one polymer selected from the group consisting of 
ethylene-propylene-butene-1 terpolymer, ethylene-propylene random 
copolymer, ethylene-propylene block copolymer, isotactic polypropylene, 
low density polyethylene (LDPE), very low density polyethylene (VLDPE), 
linear low density polyethylene (LLDPE), medium density polyethylene 
(MDPE), said polymer being compounded with i) an effective amount of 
anti-blocking agent and ii) a quantity of silicone oil such that a 
coefficient of friction-reducing amount is present on the exposed surface 
of layer (c) as well as the exposed surface of layer (a) after mutual 
contact of said surfaces. 
In another aspect, the present invention relates to a method of making a 
film which comprises: 
(1) providing the upper major surface of the above-described core layer (b) 
derived from a high density polyethylene with a coextensive water-based 
coating receiving upper skin layer (a) formed from at least one polymer 
selected from the group consisting of ethylene-propylene-butene-1 
terpolymer, ethylene-propylene random copolymer, ethylene-propylene block 
copolymer, isotactic polypropylene, low density polyethylene (LDPE), very 
low density polyethylene (VLDPE), linear low density polyethylene (LLDPE), 
medium density polyethylene (MDPE), polyvinylidene chloride (PVDC), 
polyvinyl alcohol (PVOH) and acrylic, said polymer being compounded with 
an effective amount of anti-blocking agent, but being substantially devoid 
of silicone oil; 
(2) providing the lower major surface of core layer (b) with a coextensive 
surface lower skin layer (c) formed from at least one polymer selected 
from the group consisting of ethylene-propylene-butene-1 terpolymer, 
ethylene-propylene random copolymer, ethylene-propylene block copolymer, 
isotactic polypropylene, low density polyethylene (LDPE), very low density 
polyethylene (VLDPE), linear low density polyethylene (LLDPE), medium 
density polyethylene (MDPE), said polymer being compounded with i) an 
effective amount of anti-blocking agent and ii) a quantity of silicone oil 
such that a coefficient of friction-reducing amount will be present on the 
exposed surface of layer (c) as well as the exposed surface of layer (a) 
after mutual contact of said surfaces; and 
(3) contacting the exposed surface of layer (c) with the exposed surface of 
layer (a) such that a coefficient of friction reducing amount of silicone 
oil is transferred from the exposed surface of layer (c) to the exposed 
surface of layer (a). 
The film composition of the present invention can comprise a biaxially 
oriented high density base layer of at least 50 weight percent of a high 
density polyethylene having a density of 0.96 or higher. The skin layers 
are used with HDPE films which have been biaxially oriented to provide 
product having good WVTR properties as well as other characteristics, 
e.g., barrier properties for gases such as oxygen, which are desirable in 
packaging films. Those embodiments wherein a skin layer consists 
essentially of PVDC, PVOH or acrylic exhibit reduced water vapor 
transmission rate and oxygen transmission rate. 
The skin layers help provide a film which is readily castable while 
maintaining the desirable properties of HDPE including dead fold, twist 
retention, TD tear and moisture barrier. The films can be cast, and 
oriented in the solid state up to about two times in the machine direction 
and six times or more in the transverse direction to give films having 
good dead-fold and water vapor transmission rate characteristics making 
them highly suitable for packaging, particularly for dry foods. 
The films with or without heat seal compositions in the skins can be used 
on a wide variety of packaging machines such as vertical form, fill and 
seal (VFFS) and high speed horizontal slug wrapping machines and can also 
be used for twist wrapping machines and laminations. 
The films of the present invention are especially suitable for applications 
requiring good dead fold characteristics. The present films can exhibit 
crease retention (as measured by ASTM D 920-49) in either or both 
directions of greater than 70%, greater than 85%, or even greater than 95% 
after 30 seconds, e.g., 70% to 85% TD and 75% to 95% MD. 
The present films can possess both good lubricity and good receptivity for 
water-based surface coating materials and optical clarity allows the 
convertor/end user to utilize inks and/or adhesives which do not 
incorporate organic solvents. Organic solvents generally present health, 
explosion and fire hazards and resort must be had to expensive ventilation 
and safety equipment to provide an acceptable working environment where 
they are used. Even with the use of such equipment, in some jurisdictions, 
the atmospheric emission of organic solvents is prohibited and expensive 
solvent recovery systems are required to comply with legal limitations. 
The films herein avoid organic solvent-containing coating materials 
thereby eliminating the health, safety and environmental problems 
attendant their use as well as the cost of the equipment required to deal 
with the hazards which they pose. 
DETAILED DESCRIPTION 
According to the present invention, the upper skin layer is derived from at 
least one polymer selected from the group consisting of 
ethylene-propylene-butene-1 terpolymer, ethylene-propylene random 
copolymer, ethylene-propylene block copolymer, isotactic polypropylene, 
low density polyethylene (LDPE), very low density polyethylene (VLDPE), 
linear low density polyethylene (LLDPE), medium density polyethylene 
(MDPE), polyvinylidene chloride (PVDC), polyvinyl alcohol (PVOH) and 
acrylic and any polymer blends being of polymers which are compatible with 
one another. It should be noted that PVDC, PVOH and acrylic are 
incompatible with the other named polymers so that the former are used 
singly. 
In one embodiment the upper skin layer (a) is derived from a random 
ethylene-propylene copolymer. This may be derived from ethylene and one or 
more co-monomers. The propylene content of these random copolymers is 
typically from about 70 to 85 weight percent, more usually from about 75 
to 85 percent, with the balance of ethylene and any other comonomers such 
as butylene. Suitable copolymers of this type are random copolymers of 
ethylene and propylene or random terpolymers of ethylene, propylene and 
butylene. Preferred copolymers of this type include the following: 
Ethylene-propylene copolymers containing 2-10 weight percent random 
ethylene, e.g. 3-7 weight percent ethylene. 
Ethylene-propylene-butylene random terpolymers containing 1-5 weight 
percent random ethylene, 10-25 weight percent random butylene. The amounts 
of the random ethylene and butylene components in these copolymers are 
typically in the range of 10 to 25 percent total (ethylene plus butylene). 
Typical terpolymers of this type include those with about 1-5 percent 
ethylene and 10-25 percent butylene. 
These copolymers typically have a melt flow rate in the range of about 5 to 
10 with a density of about 0.9 and a melting point in the range of about 
115.degree. to about 130.degree. C. 
In another embodiment the upper skin layer (a) is derived from a low 
density polyethylene. This may be a linear low density polyethylene 
(LLDPE) or a non-linear polyethylene. These polymers typically have a melt 
index of 1 to 10. The low density polyethylenes should have a density of 
0.88 to 0.93 while the linear materials may have a density as high as 
0.94, usually in the range 0.90-0.94, e.g. 0.918 or 0.921, with a melt 
index from about 1 to about 10. The linear low density polyethylenes may 
be derived from ethylene together with other higher comonomers such as 
hexene-1 or octene-1. 
Prior to incorporation in the film, e.g., before extrusion, upper layer (a) 
is compounded with an anti-blocking effective amount of an anti-blocking 
agent, e.g., silica, clays, talc, glass, and the like, which are 
preferably provided in the form of approximately spheroidal particles. The 
major proportion of these particles, for example, anywhere from more than 
half to as high as 90 weight percent or more, will be of such a size that 
significant portion of their surface area, for example, from about 10 to 
70 percent thereof, will extend beyond the exposed surface of layer (a). 
Although the polymer from which layer (a) is formed is not compounded with 
a silicone oil, this layer will ultimately acquire a coefficient of 
friction-reducing amount of silicone oil. Thus, when the finished film 
laminate containing silicone oil on the exposed surface of layer (c) is 
taken up on a winding coil, some of the silicone oil will be transferred 
from this surface to the exposed surface of layer (a), primarily to the 
exposed surfaces of the anti-blocking agent particles which protrude from 
layer (a). However, since the interior of layer (a) contains no amount of 
silicone oil which could interfere with the heat sealing or other 
properties of this layer (and ordinarily contains no silicone oil at all), 
the presence thereon of the transferred silicone oil serves to reduce the 
coefficient of friction of the layer without significantly impairing its 
receptivity to water-based coatings, its heat sealability or its optical 
clarity. 
Optionally, layers (a) and/or (c) can contain a minor amount of a wax, 
e.g., a microcrystalline wax for added lubricity and/or reduced water 
vapor transmission rate. Amounts of wax of from about 2 to about 15 weight 
percent of either or both layers (a) and (c) can be used if desired. 
Either or both of these layers can also contain pigments, fillers, 
stabilizers, light protective agents or other suitable modifying 
ingredients if desired. 
In order to enhance its receptivity for water-based coatings and inks, 
layer (a) can be treated by such known and conventional techniques as 
corona discharge and flame treating. 
Lower layer (c) of the film laminate can be of substantially the same 
composition as that of upper surface layer (a) except that i) the polymer 
is not derived from components selected from the group consisting of PVDC, 
PVOH and acrylic and ii) the polymer constituting layer (c) is compounded 
with a silicone oil. The silicone oil advantageously possesses a viscosity 
of from about 350 to about 100,000 centistokes with 10,000 to about 30,000 
centistokes being especially preferred. Examples of suitable silicone oils 
are polydialkylsiloxanes, polyalkylphenylsiloxanes, olefin-modified 
siloxane oils, olefin/-polyether-modified silicone oils, epoxy modified 
silicone oils and alcohol-modified silicone oils, polydialkylsiloxanes 
which preferably have from about 1 to about 4 carbon atoms in the alkyl 
group, in particular polydimethylsiloxanes. Of the foregoing, the 
polydialkylsiloxanes, in particular a polydimethylsiloxane, are preferred 
for use herein. 
The silicone oil can be added to lower skin layer (c) generally in the form 
of a dispersion or emulsion, the silicone being present within, as well as 
on the exposed surface of this layer as discrete microglobules, frequently 
of an average size of from about 1 to about 2 microns. The silicone oil, 
which is generally substantially uniformly distributed on the exposed 
surface of layer (c), is responsible for imparting a reduced coefficient 
of friction to this surface as well as to the exposed surface of layer (a) 
when some of the oil is transferred thereto after these surfaces have been 
placed in mutual contact, e.g., as will occur when the laminate film has 
been wound on a winding coil. 
Polydimethylsiloxane or other silicone oil can be present at from about 
0.15 to about 1.5 weight percent of lower layer (c). Some of this silicone 
oil will, of course, be present on the exposed surface of layer (c). The 
amount selected should in any event be sufficient to provide a coefficient 
of friction of layers (a) and (c) (following transfer of silicone oil 
microglobules to the latter) of about 0.4 or less, preferably between 
about 0.25 to about 0.3 up to at least about 60.degree. C. Because of the 
manner in which the silicone oil is applied to just the exposed surface of 
upper layer (a), such layer exhibits an improved coefficient of friction 
but not at the expense of its receptivity to water-based coatings, its 
heat sealability or its optical clarity. 
The silicone oil should be incorporated as homogeneously as possible in the 
polymer constituting layer (c). This can be achieved by either 
incorporating the silicone oil as a dispersion or emulsion at room 
temperature and then heating the blend with the application of shearing 
forces or by incorporating the oil while the blend is being melted. The 
mixing temperature must be high enough to soften the blend and enable a 
very uniform mixture to be formed. The temperature required in a kneader 
or extruder is generally from about 170.degree. to about 270.degree. C. 
Core layer (b) is derived from imbalanced biaxially oriented film base 
layer made from a major proportion of a high density polyethylene (HDPE) 
having a density of at least 0.96. The film can be composed exclusively of 
a single HDPE resin, a mixture of HDPE resins, or of HDPE containing a 
minor proportion of other resource polymers. These high density 
polyethylenes typically have a melt index from about 0.5 to about 10, 
usually from about 0.7 to 2. The mixture of HDPE resins gives better 
processing characteristics in the extruder by reducing extruder torque. 
Films made with a blend of HDPE resins reduce splittiness of the film 
which manifests itself as the tendency of the film to break in the TD 
direction during operation on vertical, form, fill and seal (VFFS) 
machinery. 
The blends of HDPE polymers can comprise two or more polymers all of which 
preferably have densities of 0.96 or greater. Blends of HDPE polymers 
advantageously comprise a major proportion of HDPE having a melt index of 
0.5 to 6 and one or more polymers having a different melt index. 
Terblends have been found particularly desirable. Suitable terblends 
generally comprise 50 to 98 weight percent, preferably 84 to 96 weight 
percent of HDPE having a density of 0.96 or higher and a melt index of 
greater than 0.5 to about 1.0; 1 to 25 weight percent, preferably 3 to 8 
weight percent of HDPE having a density of 0.96 or greater and a melt 
index of 0.1 to 0.5; and 1 to 25 weight percent, preferably 3 to 8 weight 
percent, of HDPE having a density of 0.96 or higher and a melt index of 
greater than 2 to about 8. Preferably, the second and third HDPE polymers 
which are minor components are present in about equal amounts. 
In a preferred embodiment, the film of the present invention has both its 
upper skin layer and lower skin layer comprising a mixture of 
ethylene-propylene-butene-1 terpolymer and low density polyethylene 
(LDPE). For example, the upper skin layer and lower skin layer can 
comprise 85 to 95 wt %, say 90 wt %, of ethylene-propylene-butene-1 
terpolymer and 5 to 15 wt %, say 10 wt % low density polyethylene (LDPE). 
The proper degree of orientation in the film contributes to the desired 
physical properties, as well as good WVTR and dead-fold characteristics. 
For example, it has been determined that films with a thickness of 1.4 to 
4 mils will have acceptable WVTR (g-mil/100 in.sup.2 /24 hr-1 atm) of less 
than about 0.2/mil whereas a somewhat heavier gauge (1.5 times thicker or 
more) is needed in a blown HDPE film to achieve the same WVTR. The 
benefits of reduced WVTR are due to the improvements obtained by biaxial 
orientation below the HDPE melting point. Although higher density HDPE 
resin having a density of 0.957 or greater can be made directly into thin 
films by cast extrusion, problems of curling, uniformity, flatness and 
high WVTR remain as obstacles. Accordingly, thin HDPE films of about 0.8 
to 1.5 mils having the best balance of properties, particularly for VFFS 
applications, are obtained with imbalanced biaxially oriented films 
prepared from films having a cast gauge of 12 to 20 mils reduced to the 
desired gauge by orientation. The final film gauge will typically be not 
more than about 2.5 mils. 
The HDPE base film is oriented either before or after the skins are applied 
to a degree of 1.1:1 to about 2:1, usually from about 1.25:1 to 2:1, in 
the machine direction (MD), and to a degree of about 6:1 to about 12:1 in 
the transverse direction (TD). The films exhibit improved dead-fold, and 
other physical properties which are markedly better than cast and blown 
HDPE films, even when the total film thickness is reduced to less than 
about 1 or 2 mils. When provided with a skin layer as described below, the 
films are particularly suitable for use in packaging, especially of dry 
foodstuffs. The films may be used in a wide variety of packaging equipment 
including vertical form, fill and seal (VFFS), high speed horizontal slug 
wrapping, as well as twist folding packaging machines. 
The skin layers can be applied to the HDPE base film in any conventional 
manner, for example, by coating or coextrusion before orientation or by 
coating the HDPE after one or both of the biaxial orientation operations. 
Generally, the skin layers of the film of the invention comprise less than 
50 wt %, preferably less than 15 wt %, say about 10 wt % of the total 
film. 
The films may be produced and oriented in the conventional manner. When the 
skin layer is present on one or both sides of the HDPE film, cast 
extrusion is generally accomplished using a standard cast roll and water 
bath system. 
In the usual manner the film is heated to its orientation temperature and 
first subjected to MD orientation between two sets of nip rolls, the 
second rotating at a greater speed than the first in an amount equal to 
the desired draw ratio. Then the film is TD oriented by heating and 
subjecting it to transverse stretching in a tenter frame. Typically MD 
orientation is conducted at 60.degree. to 120.degree. C. and TD 
orientation at 110.degree. to 160.degree. C. 
The skin layers and/or base layer may contain stabilizers to minimize 
oxidative and thermal degradation, as well as other additives to achieve 
other functionality including, but not limited to, static reduction, ease 
of processing, ink receptivity etc.

The invention is illustrated by the following non-limiting examples in 
which all parts are by weight unless otherwise specified. 
EXAMPLE 
A three-layer biaxially oriented film having a 1.15 mil final thickness was 
prepared by coextruding: 
a base layer HDPE (M-6211 obtained from Oxychem of Houston, Tex.) 
comprising 90% of the total film thickness; 
a first skin layer on one side of the base layer comprising 5% of the total 
film thickness which is Chisso 7510, obtained from Chisso Corp. of Japan, 
consisting of ethylene-propylene-butene-1 terpolymer, the first skin layer 
being subjected to treatment by corona discharge; and 
a second skin layer on the other side of the base layer (Chisso 7820, 
obtained from Chisso Corporation of Japan), comprising 5% of the total 
film thickness, whose resin component is a blend of about 90% 
ethylene-propylene-butene-1 terpolymer and 10% LDPE which second skin 
layer further contains about 1 wt % of silicone oil. 
The film was prepared in a standard cast roll system and then oriented 1.3 
times in the MD at about 115.degree. C., and 9.0 times in the TD direction 
at 110.degree.-160.degree. C. in a tenter frame. 
The film was tested for dead fold properties by ASTM D-920-49 (% crease 
retention after 30 seconds). In the transverse direction, the film of the 
present invention (HDPE with terpolymer skins) exhibited about 75% crease 
retention, and about 85% crease retention in the machine direction. This 
compares favorably with 0.80 mil polypropylene film with terpolymer skins 
(80 BSR-ONE available from Mobil Chemical Co. of Pittsford, N.Y. (8% TD, 
6% MD)); 1.25 mil 100% polypropylene film (240 B available from Mobil 
Chemical Co. of Pittsford, N.Y. (15% TD, 13% MD)); 1.85 mil HDPE film with 
ionomer skin (Blown HD, available from American National Can Co. of 
Neenah, Wis. (47% TD, 36% MD)); and 2.0 mil wax coated glassine (55%).