Metal bond strength in polyolefin films

The present invention relates to the improvement of metal bonding strength in polypropylene films through the addition of ethylene in a mini-random ethylene-propylene copolymer in an amount of no more than about 1 weight percent, more preferably no more than about 0.7 weight percent, and most preferably between about 0.3 weight percent and about 0.5 weight percent, or even amounts between about 0.05 weight percent and about 0.2 weight percent. The invention allows the improvement of metal bond strength in metallizable films. The invention encompasses both the resulting films with enhanced metal bond strength and the process for producing such films. In the preferred embodiment, the proposed mini-random copolymer is formed into a film layer used in place of a propylene homopolymer layer, providing improved bonding properties over a simple polypropylene homopolymer, while maintaining at acceptable levels the physical and optical characteristics of a film layer made from a propylene homopolymer, such as stiffness.

FIELD OF THE INVENTION 
This invention relates to metallizable polyolefin films, and more 
particularly to the improvement of metal bonding properties of such films. 
BACKGROUND OF THE INVENTION 
For some packaging films, the barrier properties are improved tremendously 
by vacuum depositing aluminum onto the surface of biaxially-oriented 
polypropylene films. Also, for decorating purposes metal deposition may be 
performed to give the film a reflective coating. Sometimes a lamination or 
other secondary process is performed which can damage the metal coating. 
Thus, a strong metal bond between the metal layer and the base or film 
layer is preferred. This preference extends more generally to other 
polypropylene films where greater metal bond strength improves the 
wear-life and quality of a deposited metal layer. 
In films where metal coating is put directly onto a homopolymer 
polypropylene after surface treatment (such as corona treating (also known 
as corona discharge treating), flame treating, etc.) the metal bond is not 
noted to be very strong. However, often the physical and optical 
properties of a homopolymer polypropylene are more desirable to the 
overall objects of the film, necessitating against the use of a standard 
ethylene-polypropylene copolymer or ethylene-butene-polypropylene 
terpolymer, or other multiple polymer system known to have good bonding 
properties. 
The polymers normally employed in the preparation of biaxially-oriented 
polypropylene films are isotactic polymers such as isotactic 
polypropylene, although on some occasions the use of syndiotactic polymers 
has been proposed. Isotactic polypropylene is one of a number of 
crystalline polymers which can be characterized in terms of the 
stereoregularity of the polymer chain. Various stereo-specific structural 
relationships denominated primarily in terms of syndiotacticity and 
isotacticity may be involved in the formation of stereoregular polymers 
for various monomers. 
Isotactic polypropylene is conventionally used in the production of 
relatively thin films in which the polypropylene is heated and then 
extruded through dies and subjected to biaxial orientation by stressing 
the film in both a longitudinal direction (referred to as the machine 
direction) and in a transverse or lateral direction sometimes referred to 
as the "tenter" direction. The structure of isotactic polypropylene is 
characterized in terms of the methyl group attached to the tertiary carbon 
atoms of the successive propylene monomer units lying on the same side of 
the main chain of the polymer. That is, the methyl groups are 
characterized as being all above or below the polymer chain. Isotactic 
polypropylene can be illustrated by the following chemical formula: 
##STR1## 
Another way of describing the structure is through the use of NMR Bovey's 
NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" 
representing a "meso" dyad, or successive methyl groups on the same side 
of the plane of the polymer chain. As is known in the art, any deviation 
or inversion in the structure of the chain lowers the degree of 
isotacticity and crystallinity of the polymer. 
The isotactic polymers normally employed in the preparation of 
biaxially-oriented polypropylene films are usually those prepared through 
the use of conventional Ziegler-Natta catalysts of the type disclosed, for 
example, in U.S. Pat. Nos. 4,298,718 and 4,544,717, both to Myer et al. 
Thus, U.S. Pat. No. 5,573,723 to Peiffer et al discloses a process for 
producing biaxially-oriented polypropylene film based on an isotactic 
polypropylene homopolymer or propylene-ethylene copolymers. Other 
copolymers of propylene and alpha-olefins having from 4-8 carbon atoms 
also may be employed in the Peiffer process. 
Catalysts employed in the polymerization of alpha-olefins may be 
characterized as supported catalysts or unsupported catalysts, sometimes 
referred to as homogeneous catalysts. Traditional supported catalysts are 
the so-called "conventional" Ziegler-Natta catalysts, such as titanium 
tetrachloride supported on an active magnesium dichloride as disclosed, 
for example, in the aforementioned patents to Myer et al. A supported 
catalyst component, as disclosed in the Myer '718 patent, includes 
titanium tetrachloride supported on an "active" anhydrous magnesium 
dihalide, such as magnesium dichloride or magnesium dibromide. The 
supported catalyst component in Myer '718 is employed in conjunction with 
a co-catalyst such as an alkylaluminum compound, for example, 
triethylaluminum (TEAL). The Myer '717 patent discloses a similar compound 
which may also incorporate an electron donor compound which may take the 
form of various amines, phosphenes, esters, aldehydes, and alcohols. 
Metallocene catalysts are often employed as unsupported or homogeneous 
catalysts, although, as described below, they also may be employed in 
supported catalyst components. 
Alternative types of catalysts that produce isotactic polyolefins are 
disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403. These patents 
disclose chiral, stereorigid metallocene catalysts that polymerize olefins 
to form isotactic polymers and are especially useful in the polymerization 
of highly isotactic polypropylene. 
SUMMARY OF THE INVENTION 
The present invention relates to metallized polyolefin film. The film 
includes a film layer formed of an ethylene-propylene copolymer where the 
ethylene is present in an amount of no more than about 1 weight percent 
and preferably between about 0.1 weight percent and about 0.7 weight 
percent. The film layer is surface treated (preferably corona treated) on 
at least one side (i.e., at least one surface), preferably to a level of 
at least about 48 dynes/cm as measured contemporaneously with treatment. 
The film layer is metallized after surface treatment with metal deposited 
on the treated surface of the film layer. The deposited metal layer has a 
thickness less than the film layer. The resulting film has a bond strength 
between the film layer and the metal layer which is at least 30 percent 
greater than the bond strength between the metal layer material and a 
correspondingly surface treated film layer formed of polypropylene 
homopolymer. The present invention further relates to a method for 
producing such a metallized film.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention involves the use of a mini-random ethylene-propylene 
copolymer in amounts of no more than about 1 percent in combination with 
propylene in the making of films with enhanced metal bond strengths. While 
applicable in most propylene films where the basic physical and optical 
characteristics of a propylene homopolymer layer are needed, but an 
enhanced bonding strength is desired, the present description focuses on 
use in biaxially-oriented polypropylene films. Those with skill in the art 
will recognize the transferability of the same enhanced bonding strength 
provided by the teachings of the invention regardless of whether a film is 
oriented in one, two, or no directions. 
Biaxially-oriented films are characterized in terms of certain well-defined 
characteristics relating to their stereoregular structures and physical 
properties, including melt temperatures and shrinkage characteristics, as 
well as in relatively low coefficients of friction and relatively high 
tensile moduli and good barrier properties including relatively low 
permeation rates to oxygen and water. The biaxially-oriented films of the 
present invention are formed using a particularly configured polyolefin 
polymer as described in greater detail below and by using any suitable 
oriented film production technique, such as the conventionally-used tenter 
frame process. 
The present invention addresses the use of a propylene-ethylene mini-random 
copolymer with an amount of ethylene no more than about 1 percent. The 
preferred method involves polymerization of the ethylene and propylene in 
the presence of an isospecific catalyst as known in the art. The resulting 
polymer incorporates the ethylene within the isotactic structure and 
pattern of the propylene. Alternative uses could incorporate the ethylene 
in an isotactic/syndiotactic polypropylene blend such as that disclosed in 
U.S. patent application Ser. No. 08/954,324, (entitled Improved Metal Bond 
Strength in Polypropylene Films, with the same inventive entity as the 
present application, filed contemporaneously with the present application, 
the entire disclosure of which is incorporated herein by reference), while 
maintaining many of the advantages of the proposed invention. 
The polymerized mixture will often further include minor amounts (typically 
less than 1 weight percent, and more typically less than 0.5 weight 
percent) of additives designed to enhance other physical or optical 
properties. Such mixtures may have, for example, one or more anti-oxidants 
present in an amount totaling no more than about 0.25 weight percent (in 
the tested examples below no more than about 0.15 weight percent) and one 
or more acid neutralizers present in an amount totaling no more than about 
0.25 weight percent (in the tested examples below no more than about 0.05 
weight percent). Although not present in the tested examples, additives 
acting as "anti-block" agents may also be present, again in relatively low 
percentages such as no more than about 1 weight percent, more preferably 
no more than about 0.5 weight percent, or alternatively no more than about 
0.25 weight percent. 
In general, biaxially-oriented film production can be of any suitable 
technique, such as disclosed in Canadian Patent Application No. 2,178,104 
to Peiffer et al. As described in the Peiffer et al application, the 
entire disclosure of which is incorporated herein by reference, the 
polymer or polymers used to make the film are melted and then passed 
through an extruder to a slot die mechanism after which it is passed over 
a first roller, characterized as a chill roller, which tends to solidify 
the film. The film is then oriented by stressing it in a longitudinal 
direction, characterized as the machine direction, and in a transverse 
direction to arrive at a film which can be characterized in terms of 
orientation ratios, sometimes also referred to as stretch ratios, in both 
longitudinal and transverse directions. The machine direction orientation 
is accomplished through the use of two sequentially disposed rollers, the 
second or fast roller operating at a speed in relation to the slower 
roller corresponding to the desired orientation ratio. This may 
alternatively be accomplished through a series of rollers with increasing 
speeds, sometime with additional intermediate rollers for temperature 
control and other functions. After the film has been stressed in the 
machine direction, it is again cooled and then pre-heated and passed into 
a lateral stressing section, for example, a tenter frame mechanism, where 
it is again stressed, this time in the transverse direction. Orientation 
in the transverse direction is often followed by an annealing section. 
Subsequently, the film is then cooled and may be subjected to further 
treatment, such as a surface treatment (for example corona treatment or 
flame treatment), as described, for example, in the aforementioned 
Canadian Patent Application 2,178,104 or in U.S. Pat. No. 4,692,380 to 
Reid, the entire disclosure of which is incorporated here by reference. 
The film may also be metallized as described in the aforementioned U.S. 
Pat. No. 4,692,380 to Reid. While corona and flame treatment typically 
occurs immediately following orientation and prior to the initial roll up, 
metallizing is typically performed at a separate time and location. 
The metal coating (which when applied forms the metal layer) may be applied 
to one or both surfaces of the film by any known method such as 
sputtering, vacuum deposition, or electroplating (all of which fall within 
the definition of "metallizing" the film and involving some act or method 
of "depositing" a metal onto the surface of the film layer). Vacuum 
deposition is a preferred method. Preferred values for the average 
thickness of the metal coating layer are within the range of about 20 to 
100 nanometers, with the preferred average thickness for the film to be 
metallized being within the range of about 0.3 microns to 150 microns. 
Regardless, the metal layer preferably has a thickness less than the film 
layer, preferably substantially less than said film layer. 
It is preferred to surface treat the surface of the film to be coated (or 
metallized) through either a corona discharge treatment or a flame 
treatment in order to improve metal bond strength. In accordance with the 
present invention, by surface treating (preferably corona treating) the 
ethylene-propylene copolymer, metal bond strength is further enhanced. 
Preferably in carrying out the present invention, the ethylene-propylene 
film layer is surface treated to a level of about 48 dynes/cm or more, 
providing the most dramatic results as illustrated by the example below. 
The most frequently used coating material is aluminum, although other 
metals such as gold, silver, and copper are also employed on occasion. It 
is recognized in the art that while the metal coating predominantly 
consists of the identified metal (such as aluminum) amounts of other 
additives may be present to improve assorted physical and optical 
properties of the deposited metal layer. In some occasions, pure aluminum 
(or the metal of choice) may be used. Other additives may be used in minor 
amounts such that aluminum (or the metal of choice) is the major 
component). Preferably aluminum (or the metal of choice) is present in the 
coating at levels of at least about 90 weight percent, at least about 95 
weight percent, and at least about 99 weight percent of the metal coating. 
Turning now to FIG. 1, there is shown a schematic illustration of a 
suitable "Tenter Frame" orientation process which may be employed in 
producing biaxially-oriented polypropylene film ("BOPP film") in 
accordance with the present invention. More particularly and with 
reference to FIG. 1, a source of molten polymer is supplied from a hopper 
10 to an extruder 12 and from there to a slot die 14 which produces a 
flat, relatively thick film 16 at its output. Film 16 is applied over a 
chill roller 18, and it is cooled to a suitable temperature within the 
range of about 30-60.degree. C. The film is drawn off the chill roller 18 
to a stretching section 20 to which the machine direction orientation 
occurs by means of idler rollers 22 and 23 which lead to preheat rollers 
25 and 26. 
As the film is drawn off the chill roller 18 and passed over the idler 
rollers, it is cooled to a temperature of about 30-60.degree. C. In 
stretching the film in the machine direction, it is heated by preheat 
rollers 25 and 26 to an incremental temperature increase of about 
60-100.degree. C. and then passed to the slow roller 30 of the 
longitudinal orienting mechanism. The slow roller may be operated at any 
suitable speed, usually about 20-40 feet per minute in this type of pilot 
production line. The fast roller 31 is operated at a suitable speed, 
typically about 150 feet per minute in a pilot line, to provide a surface 
speed at the circumference of about 4-7 times that of the slow roller in 
order to orient the film in the machine direction. In a commercial 
production line, casting speeds may be much higher such as 20 to 60 meters 
per minute, with 120 to 360 meters per minute in final speeds. 
As the oriented film is withdrawn from the fast roller, it is passed over a 
roller 33 at room temperature conditions. From here it is passed over 
tandem idler rollers 35 and 36 to a lateral stretching section 40 where 
the film is oriented by stretching in the transverse direction. The 
section 40 includes a preheat section 42 comprising a plurality of tandem 
heating rollers (not shown) where it is again reheated to a temperature 
within the range of 130-180.degree. C. From the preheat section 42 of the 
tenter frame, the film is passed to a stretching or draw section 44 where 
it is progressively stretched by means of tenter clips (not shown) which 
grasp the opposed sides of the film and progressively stretch it laterally 
until it reaches its maximum lateral dimension. Lateral stretching ratios 
are typically greater than machine direction stretch ratios and often may 
range anywhere from 5-12 times the original width. Ratios of 8-10 times 
are usually preferred. The concluding portion of the lateral stretching 
phase includes an annealing section 46, such as an oven housing, where the 
film is heated at a temperature within the range of 130-170.degree. C. for 
a suitable period in time, about 1-10 seconds. The annealing time helps 
control certain properties, and increased annealing is often specifically 
used to reduce shrinkage. The biaxially-oriented film is then withdrawn 
from the tenter frame and passed over a chill roller 48 where it is 
reduced to a temperature of less than about 50.degree. C. and then applied 
to take-up spools on a takeup mechanism 50. From the foregoing 
description, it will be recognized that the initial orientation in the 
machine direction is carried out at a somewhat lower temperature than the 
orientation in the lateral dimension. For example, the film exiting the 
preheat rollers is stretched in the machine direction at a temperature of 
about 120.degree. C. The film may be cooled to a temperature of about 
50.degree. C. and thereafter heated to a temperature of about 160.degree. 
C. before it is subject to the progressive lateral dimension orientation 
in the tenter section. 
The following examples illustrate the unexpected advantages in metal bond 
strength at increased levels of surface treatment. 
EXAMPLE 1 
Resins with and without a mini-random ethylene-propylene copolymer were 
processed through a biaxially-oriented polypropylene film making process 
using a tenter frame system and the resulting properties then measured. 
The trial was conducted in a sixty inch continuous pilot tenter line. The 
line was capable of 76.2 meter per minute output and two sided corona 
discharge treatment. Biaxial orientation of flat films was carried out in 
two sequential steps. The casted sheet chilled on a rotating cold steel 
roll was first stretched longitudinally (in the machine direction or "MD") 
in the tangential gap between sets of rolls rotating at different speeds. 
Subsequently, the film was stretched transversely (in the transverse 
direction or "TD") in a tenter frame in which the edges of the film were 
gripped by a series of clips and diverged in TD. Standard MD draw ratio 
was 5 in one stage (5x:1x) and that in TD was consistently 9 (1x:9x). The 
films were surface treated by means of corona discharge treatment (corona 
treatment) with the level of treatment measured contemporaneously closely 
following the treatment. Temperature settings are listed in Table 1 below: 
TABLE 1 
__________________________________________________________________________ 
Cast 
Chill 
MDO TDO 
Temp 
Melt 
Roll 
Roll 
Cond 
Stretch 
Anneal 
Cond 
Stretch 
Anneal 
__________________________________________________________________________ 
.degree.C. 
221 43 
49 
116 
121 127 166 
160 154 
.degree.F. 
430 110 
120 
240 
250 260 330 
320 310 
__________________________________________________________________________ 
This process was used to produce two monolayer BOPP film samples. Sample 
CS-1 was a controlled sample containing isotactic polypropylene generated 
using standard Ziegler-Natta catalysis and further including the following 
additives: Irganox 1010 (an anti-oxidant) in an amount of 0.1 weight 
percent, Irgafos 168 (an anti-oxidant) in an amount of 0.05 weight 
percent, and calcium stearate (an acid neutralizer) in an amount of 0.05 
weight percent. Sample RE-1 was configured identically with the sole 
exception that the polypropylene homopolymer was replaced by a 
propylene-ethylene copolymer containing approximately 0.6 percent by 
weight ethylene. 
The metal bond strength of each of the proceeding sample films was 
evaluated as follows. The mono-layer films were produced and oriented and 
then corona-treated at either a "low level" (i.e. 43-47 dynes/cm) or a 
"high level" (i.e. 52-56 dynes/cm). After production, orientation, and 
corona treatment, the films were then metallized by vacuum depositing 
aluminum metal onto one surface of the films. The films were then 
extrusion laminated with LDPE (low density polyethylene) on the metallized 
side to another film. The resulting lamination was peeled apart in an 
Instron. Because of its higher bond strength with polyethylene than with 
polypropylene, the metal usually adheres to the polyethylene. Thus, the 
strength required for delamination is the measure of the bond strength of 
the metal to the polypropylene substrate (the base film). Even if failure 
were to occur in a different mode, the results would still constitute a 
minimum boundary for the strength of the polypropylene to metal bond, as 
that bond would not yet have failed when the alternative failure mode 
occurred. 
The results are summarized in Table 2 below: 
TABLE 2 
______________________________________ 
Comparative 
Example 1 
Example 
______________________________________ 
Resin Label CS-1 RE-1 
ethylene, weight percent 
0.0 0.6 
Lamination Bond Strength 
Maximum, N/m 54 87 
High Level Corona! 
Averaged, N/m 39 66 
High Level Corona! 
Maximum, N/m 90 75 
Low Level Corona! 
Averaged, N/m 59 56 
Low Level Corona! 
______________________________________ 
These results demonstrate that the use of an ethylene-propylene copolymer 
including 0.6 percent by weight ethylene in films corona treated at a 
level between 52-56 dynes/cm provides an improvement in the metal bond 
strength averaging about 70 percent greater than the metal bond strength 
of the same metal layer with a correspondingly surface treated (i.e., 
surface treated using the same method of treatment to about the same 
level) film formed of non-blended polypropylene homopolymer (i.e., without 
the addition/presence of ethylene). Further, when the films were corona 
treated to a level between 52-56 dynes/cm, the maximum bond strength 
measured for the mini-random ethylene-propylene copolymer was about 60 
percent greater than the metal bond strength of the same metal layer with 
a correspondingly surface treated film formed of non-blended polypropylene 
homopolymer. Additional improvement of strength would be anticipated with 
ethylene percentages of up to about 0.8 and even about 1 weight percent, 
but above about 1 percent it is believed that the deterioration of 
physical and optical properties of the copolymer will outweigh the 
potential benefits of continued metal bond strength improvement. 
Review of the testing at lower levels of corona treatment (43-47 dynes/cm 
as measured contemporaneously with treatment) reveal that the advantages 
gained provide an unexpected trend in metal bonding strength at the 
increased levels of corona treatment. The control sample of polypropylene 
homopolymer shows a decrease in bond strength averaging 33 percent and a 
decrease in the maximum bond strength of 40 percent as its corona 
treatment is raised from the "low level" to the "high level." In marked 
contrast, the propylene-ethylene copolymer incorporating small percentages 
of ethylene shows marked increases in metal bond strength. These results 
demonstrate that, contrary to the normal trend, where greater metal bond 
strength is desired at increasing levels of surface treatment (preferably 
corona treatment) above 43-47 dynes/cm such as at least about 48 dynes/cm, 
between about 48-56 dynes/cm or more preferably between about 52-56 
dynes/cm (all as measured contemporaneously with treatment), the addition 
of small percentages of ethylene is particularly advantageous. 
Additionally, related information disclosed in U.S. patent application Ser. 
No. 08/953,523, (entitled Improved Heat Seal Strength in Polyolefin Films, 
with the same inventive entity as the present application, filed 
contemporaneously with the present application, the entire disclosure of 
which is incorporated herein by reference), suggests that increased 
bonding benefits without significant changes in physical or optical 
properties from those observed for propylene homopolymer may be obtained 
using surprisingly low levels of ethylene such as between about 0.1-0.2 
weight percent, or even as low as about 0.05 weight percent. Thus the use 
of no more than about 1.0 weight percent, preferably between about 
0.05-0.8 weight percent, more preferably between about 0.1-0.7 weight 
percent, and most preferably between about 0.3-0.5 weight percent, is 
believed to provide enhancement of metal bond strength of at least about 
40 percent over the strength of the bond between the metal material and a 
correspondingly surface treated film formed of polypropylene homopolymer. 
For ethylene content between about 0.5-0.7 weight percent, and 
specifically about 0.6 weight percent these advantages are anticipated to 
be at least about 50 percent, and potentially to average at least about 60 
percent. Particularly at the lower levels of ethylene content, 
specifically between about 0.05-0.4 weight percent, more specifically 
between about 0.1-0.2 weight percent, it is believed that the advantages 
in metal bond strength may extend to even lower levels of corona treatment 
than that shown in the test results above. Enhancements may only be in the 
range of about 15 percent to about 30 percent over the bond strength using 
a film formed of polypropylene homopolymer, or they may even extend to at 
least about 40 percent or even at least about 50 percent improvements 
nearing those seen at the 0.6 weight percent ethylene of the example. 
Having described specific embodiments of the present invention, it will be 
understood that modifications thereof may be suggested to those skilled in 
the art, and it is intended to cover all such modifications as fall within 
the scope of the appended claims.