Multilayer film with metallized surface

A metallized oriented film combination having a propylene polymer substrate with a high density polyethylene skin layer on at least one side thereof and a thin metal layer deposited on the surface of the high density polyethylene.

BACKGROUND OF THE INVENTION 
This invention relates to a multi-layer film having a metallized surface 
having superior metal adhesion and excellent fracture resistance. 
The bonding of metals such as aluminum, silver, chromium, etc. to plastic 
films has allowed such films to replace such metallic foils in many 
instances. The flexibility of the films necessitate the formation of a 
strong metal-plastic bond, and a number of approaches have been developed 
for providing such bonding. 
In U.S. Pat. No. 4,345,005, the disclosure of which is incorporated herein 
by reference in its entirety, a homopolymer polypropylene core layer is 
co-extruded with an ethylene propylene copolymer. The film is biaxially 
oriented and the copolymer layer is corona treated. A metal coating is 
deposited on the corona treated layer by a suitable process such as vacuum 
deposition. In order to enhance adhesion between the metal and the plastic 
film neither the core layer, nor the outer layers contain a slip agent in 
an amount which would deleteriously affect the bond between the metal and 
plastic film. 
Other approaches to providing a good metallizable surface have included 
modifying the surface of a polyolefin film with various oxidation 
processes and/or applying an adhesive to the surface of the film. Various 
primer coatings have been employed for this purpose, including 
carboxylated butadiene polymers and maleic anhydride modified static 
propylene polymers. 
It is an object of the present invention to provide a multi-layer polymeric 
film having a metallized layer strongly bonded thereto. It is another 
object of the present invention to provide a metallized film having 
excellent metal fracture resistance. Yet another object of the invention 
is to provide a metallized film having low water vapor and oxygen 
transmission rates. 
SUMMARY OF THE INVENTION 
The present invention relates to a metallized oriented thermoplastic film 
combination comprising a propylene homopolymer or copolymer substrate (B) 
having a high density polyethylene skin layer (A) on at least one side 
thereof, said polyethylene having a thin metal layer deposited thereon. In 
a preferred form of the invention the structure includes a heat sealable 
polymer layer (C) on one side of substrate (B). It is preferred that the 
polymer layers are co-extruded.

DETAILED DESCRIPTION OF THE INVENTION 
The propylene homopolymers contemplated by the present invention are from 
80-100% isotactic and preferably from 95-96% isotatic polypropylene. They 
have a melt index ranging from about 2 to about 10 grams/10 minutes and 
preferably a melt index of from about 3.5-6 grams/10 minutes. Preferred 
propylene copolymers include 98-96/2-4 propylene/ethylene copolymers and a 
50/50 propylene/butene-1 copolymer, a 95/5 propylene/1-pentene copolymer, 
a 90/10 propylene/1-hexene-copolymer, a 80-20 propylene/4-methyl-1-pentene 
copolymer, etc. 
The contemplated high density polyethylene includes polyethylenes having a 
density of about 0.960 or greater. The high density polyethylene can be 
composed exclusively of a single high density polyethylene resin, a 
mixture of high density polyethylene resins or of high density 
polyethylene containing a minor proportion of about 10 wt.% 
microcrystalline wax. High density polypropylenes, as described in U.S. 
Pat. No. 4,870,122 issued to P.C. Lu, the disclosure of which is 
incorporated herein in its entirety, can be employed herein. 
The heat sealable polymer (C) can be any layer or coating which will permit 
the film to be heat sealed to itself or heat sealed to some other surface 
The preferred heat sealable polymer is an ethylene propylene copolymer 
containing from about 2-4% by weight ethylene and from about 96% to about 
98% by weight polypropylene 
The present invention also relates to a method of producing an oriented 
propylene homopolymer or copolymer base film for metallization which 
exhibits superior metal adhesion in comparison to other oriented base 
films. This is achieved by co-extruding the high density polyethylene skin 
layer onto the selected propylene homopolmyer or copolymer base and 
subsequently orienting the structure in the machine and transverse 
directions. Thereafter, the high density polyethylene surface is treated 
either by corona discharge treatment or by flame treatment. The corona 
discharge treatment can be to about 35-60 dynes/cm. and preferably to 
about 35-40 dynes/cm. Thereafter, the base film produced can be metallized 
by any known methods for example electroplating, sputtering and vacuum 
metallizing. A preferred method of metallizing the treated outer layer is 
by vacuum deposition. The metal coating can be any of the typical metals 
such as aluminum, copper, silver and chromium. 
The metallized film produced exhibits excellent performance in conversion 
applications where a strong metal to base interface is critical. In 
adhesive and extrusion lamination applications, the metallized film 
exhibits superior bond strength and an absence of metal pick-off This is 
believed to be equal to or better than other quality metallized propylene 
polymer films produced by the prior art. For example, in extrusion 
lamination at typical process conditions, the metallized propylene polymer 
film exhibits metallized film to low density polyethylene bond strengths 
in excess of 200 g/in. with 0% metal transfer from the metallized 
propylene polymer to the low density polyethylene. 
In addition, it has been found that the film can be made metal fracture 
resistant in extrusion lamination by down gauging the thickness of the 
high density polyethylene skin layer. More specifically, for a total film 
thickness of from about 50-100 gauge units, if the high density 
polyethylene skin thickness is less than or equal to about 2 gauge units, 
the metallized high density polyethylene surface will exhibit excellent 
metal fracture resistance when contacted with a low density polyethylene 
melt during extrusion lamination. 
EXAMPLES 
Various film constructions were produced which consisted of ABC type 
structures where total film thicknesses were held constant at 70 gauge. 
The C-layer represents the sealant side and was kept constant at 4 gauge 
units. High density polyethylene (Type 7835 by Cain Chemical-3.0 M.I, 
density 0.960) was co-extruded onto a polypropylene homopolymer B-layer to 
form the A-layer. The A-layer was varied in thickness from 1.0-3.0 gauge 
units. The high density polyethylene surface of the film was treated to an 
off-line level of 43 dynes/cm. 
Films with 1.0-3.0 gauge units of high density polyethylene on the treated 
surface were vacuum metallized with aluminum, using conventional 
techniques, to an optical density of 1.8 to 2.5. To assess the degree of 
adhesion between the aluminum and the high density polyethylene A-layer 
for the films a 3M Company 610 tape test was performed on each film. The 
percentage of aluminum pulled off the surface of each metallized film 
after 3 consecutive pulls with the 610 tape was noted, and is reported 
below. 
The metallized films were subsequently extrusion laminated to an oriented 
polypropylene slip film using a low density polyethylene (10 lb./ream) at 
a melt temperature of 620.F. The metallized coextruded films were measured 
for lamination bond strength to low density polyethylene and percent metal 
transfer from the metallized surface. Metal fracture was also inspected 
for each lamination with the light scope at 25X. Results of the evaluation 
are presented in the following Table. Also shown for comparison purposes 
are results typical of metallization directly on a treated polypropylene 
homopolymer layer. 
______________________________________ 
% 
Pick- Metallized 
% Alumi- 
A-Layer Off Aluminum OPP to num 
Ex- Skin with LDPE Bond 
Transfer Metal 
am- Thickness 610 Strength to LDPE on 
Fracture 
ples (Ga. Units) 
Tape (g/in.) Bond Pull 
(25X) 
______________________________________ 
1 3.0 40 120 95 None 
2 1.0 0 330 0 None 
3 1.5 0 415 0 None 
4 2.0 0 305 0 None 
5 2.5 0 330 0 Medium 
6 3.0 0 390 0 Heavy 
______________________________________ 
As shown by the Table, 40% of the aluminum was picked off the treated 
polypropylene during the tape test. However, employing the treated high 
density polyethylene surface, no aluminum was picked-off. The Table shows 
the bond strength of the metallized oriented polypropylene to low density 
polyethylene to be 120 grams/in., which is significantly lower than the 
bond strength for Examples 2-6. The Table also reveals that using 
homopolymer polypropylene alone as the base for the aluminum, 95% of the 
aluminum transferred to the surface of the low density polyethylene. On 
the other hand, no aluminum transferred to the low density polyethylene 
for all of the Examples of the present invention. The final column of the 
Table shows that so long as the gauge thickness of the high density 
polyethylene is 2 gauge units or less, then no metal fracture occurs. When 
the gauge is greater than 2.5 the metal fracture ranges from medium to 
heavy. 
The foregoing results show that the employment of a high density 
polyethylene surface on the propylene polymer base layer provides an 
outstanding surface for bonding a metallized layer thereto. 
Although the present invention has been described with preferred 
embodiments, it is to be understood that modifications and variations may 
be resorted to, without departing from the spirit and scope of this 
invention as those skilled in the art will readily understand. Such 
variations and modifications are considered to be within the purview and 
scope of the appended claims.