Polymeric films

Biaxially oriented polyolefin films comprising a core layer of propylene polymer, an intermediate layer of a non-voided, substantially non-pigmented propylene polymer on the core layer, and an outer skin layer of a polyolefin including titanium dioxide as a pigment are disclosed. Such films have good gloss.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns polymeric films and more particularly polypropylene 
films having good gloss. 
2. Description of Related Art 
It has been proposed hitherto in EP0004633-A to produce opaque 
polypropylene films, the opacity resulting from microvoids within a core 
layer of a three layer film structure. The outer heat sealable layers of 
these films impart a degree of gloss to the films. However, higher degrees 
of opacity combined with high gloss values have been sought in the art. 
One proposal for increasing the opacity of polypropylene films has been to 
use titanium dioxide as a filler. GB2195947-A, for example, describes 
biaxially oriented polypropylene films having a base layer of a voided 
propylene homopolymer with an outer heat sealable polymeric layer 
containing titanium dioxide thereon. The titanium dioxide in the outer 
layer imparts improved opacity to the films compared with analogous voided 
films such as are described in EP 0004633-A, but the surface gloss of the 
films is reduced. 
It has been proposed in EP 0517109-A to use a layer of a propylene 
homopolymer containing finely divided titanium dioxide between a propylene 
homopolymer core layer and a polymeric outer layer containing no filler, 
and such films are said to have high gloss. 
SUMMARY OF THE INVENTION 
According to the present invention there are provided biaxially oriented 
polyolefin films comprising a core layer of a propylene polymer, an 
intermediate layer of a non-voided, substantially non-pigmented propylene 
polymer on the core layer, and an outer skin layer of a polyolefin 
including titanium dioxide as a pigment. 
It has surprisingly been found that by including titanium dioxide in the 
outer skin layer, films with particularly high gloss can be achieved 
compared, for example, with films having titanium dioxide in the 
intermediate layer and no titanium dioxide in the outer skin layer. This 
is also particularly surprising since films analogous to films of the 
present invention but without the intermediate layer between the core 
layer and a titanium dioxide containing skin layer show low gloss. The low 
gloss of these latter films is not surprising since the use of titanium 
dioxide in the outer surface of polyolefin films has been proposed 
hitherto for making synthetic papers with writable surfaces, for example 
J49012815 and J61228053. 
The core layer of films of the present invention is of a propylene polymer, 
and it is preferably of a propylene homopolymer or a copolymer of 
propylene with low amounts, e.g. up to 2 wt %, of ethylene. The core layer 
can be voided or non-voided, and known organic or inorganic voiding agents 
can be used to effect voiding using known methods, for example using 
polymeric particles e.g. polyesters such as polyethylene terephthalate or 
polybutylene terephthalate, or polyamides such as nylons, or inorganic 
particulates such as chalk. The voiding agent should be of a size such 
that it can initiate voids when the films of the present invention are 
biaxially oriented. 
In addition to or alternatively to a voiding agent, the core layers of 
films of the present invention can include one or more pigments, a 
particularly preferred pigment being titanium dioxide. 
The core layer will usually include one or more additives which impart 
antistatic and slip properties to the films, examples of materials for 
imparting such properties being known in the art. 
The intermediate layer is of a non-voided, substantially non-pigmented 
propylene polymer. The preferred propylene polymers are propylene 
homopolymers or copolymers of propylene with low amounts, e.g. up to 2 wt 
%, of ethylene. It is also preferred that the propylene polymers have an 
isotactic content of at least 90%. 
The outer skin layer is of a polyolefin, and preferably a propylene 
polymer. However, it is generally preferred that the outer skin layer 
should be heat sealable, for example to itself, and it is therefore 
preferred to use copolymers derived from two or more a-olefins including 
ethylene. For example, the polyolefin of the outer layer can be a 
copolymer containing units derived from propylene and one or more of 
ethylene, butene-1 and a-olefins containing from 5 to 10 carbon atoms. The 
outer layer can also be formed from blends of polyolefins. However, when 
blends are used, it is generally preferred that the components of the 
blend are compatible so that they do not reduce the surface gloss of the 
outer layer. 
The amount of titanium dioxide in the outer skin layer will usually be at 
least 2.5 wt % of the skin layer in order to pigment the films. However, 
it is generally preferred that the skin layer should not contain more than 
25 wt % of titanium dioxide based on the weight of the skin layer as very 
high levels of titanium dioxide can reduce the surface gloss of the films 
and it can reduce the mechanical strength of the outer skin layer, for 
example it can reduce the seal strength of heat seals formed by the films. 
Films in accordance with the present invention can be made in a variety of 
thicknesses. However, it is generally preferred that the outer skin layer 
containing titanium dioxide is in the range of from 0.5 to 8.0 .mu.m 
thick, and more preferably from 1.0 to 3.0 .mu.m thick. The intermediate 
layer of non-voided, non-pigmented propylene polymer will usually be 
thicker than the outer skin layer, preferred thicknesses being from 2 to 
15 .mu.m, with thicknesses of from 3 to 8 .mu.m being particularly 
preferred. 
The titanium dioxide pigment used in the outer layer of films in accordance 
with the present invention should be of a particle size which does not 
initiate voiding of the polymer in which it is present, the mean particle 
size preferably being less than 1 .mu.m. Particles of titanium dioxide of 
this size are usually insufficient to cause voiding of propylene 
homopolymers. 
The core layer and any other layers present can then be of a thickness 
which provides a film with the desired thickness, for example in the range 
of from 25 to 80 .mu.m. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Although films in accordance with the present invention have the three 
specified layers, it is generally preferred that they include at least one 
further layer on the surface of the core layer remote from the specified 
intermediate and outer skin layers. Such further layer or layers are 
preferably of one or more polyolefins, the outer surface formed by such 
further layer or layers preferably being of a heat sealable polyolefin. 
Thus films in accordance with the present invention preferably consist of 
four or five layers, the further layer or layers being selected to provide 
the films with properties required for that side of the films. A 
particularly preferred film structure consists of five layers, a central 
core layer with two intermediate layers of non-voided, unpigmented 
propylene polymer(s) and two outer skin layers of propylene polymer, at 
least one containing titanium dioxide. The polymers used for the two 
intermediate layers and for the two outer skin layers can then be the same 
or different on each side of the core layer, and the amount of titanium 
dioxide in the two outer skin layers can also be the same or different. 
In general, in addition to the titanium dioxide present in the outer skin 
layer or layers, films in accordance with the present invention will 
usually include an antiblock agent in both of their outer surfaces. 
Suitable antiblock agents are known in the art, for example silica. 
Films in accordance with the present invention can be produced by known 
methods, for example by coextrusion of the respective layers followed by 
simultaneous or sequential biaxial stretching. Where voiding agents, 
pigments, for example the titanium dioxide used in the outer layer(s), or 
other additives, such as antistatic or slip additives, are incorporated 
into one or more layers of the films, they are preferably introduced in 
the form of one or more masterbatches. 
It is particularly preferred to form films of the present invention by 
coextruding through a slot die melts of the polymers for the respective 
layers, preferably using masterbatches to introduce additives for the 
respective layers, and then to effect sequential stretching of the 
coextrudate. Stretching in the direction of extrusion is preferably 
effected by from 4:1 to 7:1 by passing the coextrudate over heated rollers 
rotating at different peripheral speeds, and then in the transverse 
direction, for example by from 5:1 to 10:1 using a stenter oven. The 
biaxially stretched film is then preferably annealed with some dimensional 
shrinkage, and the outer skin layer containing titanium dioxide is then 
preferably treated to increase its surface energy, for example by flame or 
corona discharge treatment, before being wound up.

The following Examples are given by way of illustration only. In all cases, 
gloss measurements are recorded for the surface of the films containing 
titanium dioxide or the surface having an intermediate layer containing 
titanium dioxide beneath the outer layer for which the measurement is 
made. 
EXAMPLE 1 
A four layer polymer web was coextruded through a slot die using melts of 
the respective polymers, the web consisting of a first outer layer of a 
propylene/ethylene copolymer (4 wt % ethylene) containing 15 wt % of 
titanium dioxide of mean particle size &lt;1 .mu.m, and 0.1 wt % of silica 
having a mean particle size of approximately 3.5 .mu.m, an intermediate 
layer of a propylene homopolymer. A core layer of the same propylene 
homopolymer as the intermediate layer, but containing 10.6 wt % of chalk 
with a mean particle size of 3 .mu.m, and a second outer layer of the 
propylene/ethylene copolymer as the first outer layer and containing the 
same amount of the same silica but without any titanium dioxide. 
The coextruded web was cooled to solidify the melts by passing it over a 
chill roll, and it was then stretched 4.5 times in the direction of 
extrusion by passing it over rollers heated at 110.degree. C., the rollers 
rotating at different peripheral speeds. Thereafter, the monoaxially 
stretched web was stretched 10 times in the transverse direction using a 
stenter oven at 163.degree. C., and the resulting biaxially stretched film 
was annealed. The cooled film was then subjected to corona discharge 
treatment on both outer faces, and the film was wound up. 
The resulting film had a total thickness of 82 .mu.m, the two outer skin 
layers being 2.5 .mu.m thick and the single intermediate layer 8 .mu.m 
thick. The remainder of the film was a core layer of voided propylene 
homopolymer. The film had a density of 0.62 g/cm.sup.3. The skin layer 
adhered well to the intermediate layer as assessed by an adhesive tape 
test. 
The RB3 gloss of the film was measured at 20.degree. using a Dr Lange 
reflectometer, and the value obtained being given in Table I. Table I also 
shows the optical density of the film. 
EXAMPLE 2 (Comparison) 
A three layer polymer film was produced using the method of Example 1 but 
with the omission of the intermediate layer. The thickness of the final 
film was 80 .mu.m, the two outer layers each being 2.5 .mu.m thick. 
The RB3 gloss of the film was measured at 20.degree. using the method used 
in Example 1, the value obtained being given in Table I with the optical 
density of the film and the adhesion of the outer layer to the core layer. 
The density of the film was 0.62 g/cm.sup.3. The skin layer did not adhere 
well to the core layer as assessed by an adhesive tape test. 
EXAMPLE 3 (Comparison) 
A four layer film was produced using the method described in Example 1 but 
with the titanium dioxide from the first outer layer being included in the 
intermediate layer in an amount of 14 wt % of the intermediate layer 
instead of in the first outer layer. The density of the film was 0.62 
g/cm.sup.3. 
The RB3 gloss of the film was measured at 20.degree. using the method used 
in Example 1, the value obtained being given in Table I with the optical 
density of the film. 
TABLE 1 
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Film Optical 
Example Thickness(.mu.m) Gloss (RB3, 20.degree.) Density 
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1 82 40 0.82 
2 (Comparison) 80 5 0.76 
3 (Comparison) 80 31 0.75 
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The gloss was measured at 20.degree. since this gives values more 
representative of gloss as assessed by the eye. The film of the present 
invention showed high gloss combined with a resistance to removal of the 
white titanium dioxide containing layer in the adhesive tape test. The 
film of the present invention also had a higher optical density than that 
of the comparison Examples. 
EXAMPLES 4-8 
Five four layer films, each being 40 .mu.m thick, were produced by the 
method described in Example 1, the respective thicknesses of the various 
layers of the resultant films being given in Table 2. However, the core 
layers of these films contained 3.5wt % of chalk having a mean particle 
size of 1 .mu.m. The density of each of the films was 0.75 g/cm.sup.3. 
EXAMPLE 9 (Comparison) 
A four layer film was produced in a similar manner to that described in 
Example 3, but using 3.5 wt % of chalk having a mean particle size of 1 
.mu.m and having the titanium dioxide in the intermediate layer rather 
than in the outer layer. The film had a density of 0.75 g/cm.sup.3. 
TABLE 2 
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Outer layer 
Inter layer 
Gloss Optical 
Example Thickness(.mu.m) Thickness(.mu.m) (RB3, 20.degree.) Density 
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4 1 3 36 0.69 
5 1 5 53 0.64 
6 2.5 3 39 0.73 
7 2.5 8 49 0.61 
8 2.5 5 52.5 0.63 
9 (Comparison) 1 3 30 0.62 
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EXAMPLE 10 (Comparison) 
A three layer polymer web was produced by coextruding a core layer of 
polypropylene homopolymer containing 11.5 wt % of titanium dioxide having 
a mean particle size of &lt;1 .mu.m with two outer layers of a 
propylene/ethylene copolymer (5 mol % ethylene) on each surface of the 
core layer. Thereafter, the web was biaxially stretched as described in 
Example 1. 
The resultant biaxially stretched film had a total thickness of 35 .mu.m, 
with each outer layer being 0.8 .mu.m thick. 
The Dr Lange 20.degree. gloss and optical density of this film are given in 
Table 3. 
EXAMPLE 11 
A four layer film was produced substantially as described in Example 1, 
except that the core layer contained 11.5 wt % of titanium dioxide having 
a mean particle size of &lt;1 .mu.m and no chalk. 
After biaxially stretching the four layer web, the film had a total 
thickness of 35 .mu.m, with each outer layer being 1 .mu.m thick. The 
intermediate layer had a thickness of 5 .mu.m, and the outer layer thereon 
contained 15 wt % of titanium dioxide. 
The 20.degree. Dr Lange gloss of the film and its optical density are given 
in Table 3. 
TABLE 3 
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Film Optical 
Example Thickness(.mu.m) Gloss (RB3, 20.degree.) Density 
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10 (Comparison) 
35 45 0.58 
11 35 66 0.65 
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