Matt biaxially stretched polypropylene film and a process for producing the same

This invention relates to matt, multilayered propylene films to which their matt appearance is imparted by a one-sided, coextruded layer of a polymer mixture.

This invention relates to matt, multilayered propylene films whose matt 
appearance is produced by least one one-sided, coextruded layer of a 
polymer mixture. 
Numerous matt, biaxially stretched polypropylene films are already known. 
Thus DE-A 3 231 013 specifically describes a polypropylene film with a 
rough surface which is produced by exposing the film to high energy 
radiation before it is stretched. 
JP 9 148 661 describes a process for producing a rough surface on a 
biaxially stretched polypropylene film by sandblasting or by chemical 
etching. 
An increasing semi-opacity obtained by the addition of organic peroxides to 
propylene/ethylene block copolymers and to copolymers based on 
polyethylene is described in JP 2 103 214. 
In DE-A 3 839 120, a rough surface is produced by 5 to 10% of inorganic 
fillers which are applied through the whole thickness of an orientated 
polypropylene film. 
A one-sidedly matt coating of inorganic and/or organic matting agent with 
an ester resin which is applied from organic solvents is described in EP-A 
262 953. 
In JP 8 038 157 and JP 3 129 264, rough surfaces are produced by .alpha.- 
and .beta.-spherulites of the polypropylene. 
Matt films in which the matt effect is produced by block copolymers of 
propylene and of ethylene or by mixing polypropylene homo- and/or 
copolymers with polyethylene are described in numerous Patents. 
Thus a non-glossy, biaxial polypropylene film which consists throughout the 
thickness of the film of a blend of polypropylene with 10 to 65% of 
polyethylene is described in JA 6 032 668. 
A multilayered film with slight surface gloss is produced by laminating a 
monoaxially stretched polypropylene film with a mixture of 
propylene/ethylene copolymer and high density polyethylene and subjecting 
the whole laminate to transverse stretching (JP 8 001 525). 
EP-A 122 495 describes an at least 2-layered biaxially stretched 
polypropylene film in which at least one layer consists of a polyolefin 
containing from 10 to 50% by weight of ethylene. An ethylene/propylene 
block copolymer having three melting ranges within the temperatures of 
from 120.degree. to 160.degree. C. is described as an example. 
EP-A 44 544 describes a multilayered, stretched polypropylene film having a 
two-layered polypropylene structure to which a mono- or biaxially 
stretched propylene/ethylene block copolymer layer is applied. The option 
of an additional layer of adhesive is left open. 
A non-glossy, laminated film produced by biaxial stretching of 
polypropylene containing from 10 to 50% of ethylene is described in JP 0 
184 840. 
DE-A-42 09 918 describes a film structure in which the matt layer consists 
of a propylene/ethylene block copolymer, a propylene/ethylene/(butyelene) 
random co- or terpolymer, an ethylene-based co- or terpolymer modified 
with polar groups and, optionally, polyamide. 
JP 0 054 783 also describes a biaxially stretched film which has a rough 
surface produced by a propylene/ethylene block copolymer. 
Matt surfaces may also be obtained by laminating one-sidedly stretched 
ethylene/propylene copolymers to a biaxially stretched polypropylene film. 
Some of the processes for the production of biaxially stretched 
polypropylene films are quite expensive or require high investment costs 
(e.g. chemical etching processes, sandblasting, electron beam hardening, 
lacquering). Other films fail to satisfy the requirements of the market in 
their optical properties, capacity for being sealed or capacity for taking 
printed matter. It was therefore an object of the present invention to 
meet the requirements of the market and to produce a matt film in existing 
plants using the already available coextrusion apparatus for biaxially 
stretched polypropylene. 
The present invention relates to matt, two-layered or multilayered, 
biaxially stretched polypropylene films comprising an outer matt layer M, 
a core layer K and selectively other layers S which are adjacent to the 
core layer on the side opposite to the matt layer, and to polymer mixtures 
from which the matt layer can be produced, characterised in that the matt 
layer M has a thickness of from 0.5 to 5.0 .mu.m and is produced from 
25-75 % of one or more polypropylene/polyethylene block copolymers, 
5-15 % of an ethylene-based co- or terpolymer modified with polar groups 
and elastomeric components, 
15-40 % of a random co- and/or terpolymer built up of 
propylene/ethylene/butylene and containing propylene as the main 
component, 
5-20 % of a high density polyethylene and 
0-2 % of a natural or synthetic silica, 
the above-mentioned components being used in the state delivered and/or in 
the form of batches of these, 
and in that the core layer K 
consists of a thermoplastic polymer and/or copolymer of propylene which may 
be equipped with conventional slip agents, antistatic agents and/or 
antiblocking agents, 
in that the layers S 
adjacent to the core layer K on the side opposite to the matt layer 
consist either of a thermoplastic polypropylene 
which may be equipped with conventional slip agents, antistatic agents 
and/or antiblocking agents 
and/or of a thermoplastic, random copolymer 
of polypropylene with one or more .alpha.-olefins having 2 to 6 carbon 
atoms, which may be equipped with conventional slip agents, antistatic 
agents and/or antiblocking agents, 
or of a layer of mixtures of ethylene-based copolymers which can be 
laminated in the heat, 
or of a combination of the above mentioned layers. 
The following may be used as starting materials for the matt layer M: 
1. Polypropylene copolymers obtained from propylene and .alpha.-olefins, 
preferably ethylene, by a process of block polymerisation or heterophasic 
copolymers of ethylene and propylene or rubber-modified polypropylene or 
PP block copolymers and reactor blends. 
2. Linear copolymers of low density ethylene which are modified and/or low 
density polyethylenes which are rubber-modified and have an anhydride 
functionality and/or terpolymers of ethylene with acrylic acid esters and 
maleic acid anhydride. 
3. Random propylene/ethylene copolymers containing 2 to 6% by weight of 
ethylene or random terpolymers of propylene, ethylene and butylene having 
a co-monomer content below 15% by weight and a butylene content below 7% 
by weight. 
4. High density polyethylene in a density range of from 0.94 to 0.965 and a 
melt index of from 0.3 to 5 g/min at 190.degree. C. under a load of 50N. 
6. Synthetic and/or natural silica/SiO.sub.2 particles having a particle 
size&lt;10 .mu.m. 
The following serve as starting materials for the core layer K: 
An isotactic polypropylene having an n-heptane soluble content of 15% by 
weight or less, a density of from 0.90 to 0.91 g/cm.sup.3 and a melt flow 
index of from 0.5 g/10 min to 8 g/10 min at 230.degree. C. under a load of 
21.2N (determined according to DIN 53735), a polypropylene having a melt 
flow index of from 1 to 4 g/10 min being particularly preferred, 
and/or 
a random propylene/ethylene copolymer containing 2 to 6% by weight of 
ethylene and having a density of, preferably, from 0.895 to 0.960 
g/cm.sup.3, a melt index of from 1 to 7 g/10 min at 230.degree. C. under a 
load of 21.2N and a crystalline melting point in the range of from 
125.degree. to 148.degree. C. (under the polarisation microscope), 
depending on the type. 
and/or 
a propylene/butylene/ethylene copolymer having a comonomer content of 
preferably&lt;15% by weight and a butylene content of preferably&lt;7% by 
weight, preferably with a melt flow index of from 0.1 to 16 g/10 min at 
230.degree. C. under a load of 21.2N, most preferably from 4 to 10 g/10 
min at 230.degree. C. and under load of 21.2N. 
The following serve as starting materials for the layers S: 
Isotactic polypropylene and/or random co- and/or terpolymers which may also 
be used as starting materials for the core layer 
and/or 
a mixture for hot laminating which in its preferred embodiment consists of 
ethylene/vinyl acetate copolymers A and at least one other copolymer 
selected from the group of ethylene/ethyl acrylate copolymers B1 or the 
group of ethylene/acrylic acid copolymers B2. 
In a preferred embodiment, the ethylene/vinyl acetate copolymer A) contains 
from 70 to 95% by weight, in particular from 75 to 95% by weight, of 
polymerised ethylene units, and in a particularly preferred embodiment the 
remainder consists mainly or completely of polymerised vinyl acetate. 
In a preferred embodiment, the ethylene/ethyl acrylate copolymer B1) 
consists substantially of polymerised ethylene units, in particular at 
least 88% by weight of polymerised ethylene units, in particular from 88 
to 92% by weight of polymerised polyethylene units, the remainder 
consisting mainly or completely of polymerised ethyl acrylate. 
In a preferred embodiment, the ethylene/acrylic acid copolymer B2) consists 
substantially of polymerised ethylene units, in particular at least 85% by 
weight of polymerised ethylene units, most preferably from 85 to 95% by 
weight of polymerised ethylene units, the remainder consisting 
substantially or completely of polymerised acrylic acid. 
The ethylene/vinyl acetate copolymer A preferably has a melt flow index of 
from 0.1 to 15 g/10 min at 230.degree. C. and 21.2N, most preferably from 
0.3 to 8 g/10 min at 230.degree. C. and 21.2N. 
Films of the type mentioned above are produced by first coextruding a 
3-layered film. After leaving the broad sheeting die, this coextruded film 
is cooled under such conditions that the matt layer cools down as slowly 
as possible. The film is then again heated to about 
100.degree.-130.degree. C. and is longitudinally stretched in a ratio of 
from 3 to 7, preferably from 4 to 5. After the longitudinal stretching, 
the film is transversely stretched by a ratio of 7 to 12, preferably 8 to 
9, at temperatures of from 150.degree. to 170.degree. C. The film is 
thermofixed before leaving the stretching tunnel. Before it is rolled up, 
it is treated at least on one side with a corona discharge or a flame 
pretreatment. 
As an alternative to the process described above for the production of a 
4-layered film, a 3-layered coextrusion of the matt layer M, the core 
layer K and, if present, a layer S is carried out, which layers are first 
longitudinally stretched all together, as described above. Between leaving 
the longitudinal stretching part and entering the transverse stretching 
part, lamination or extrusion with another layer S takes place as 
described in EP 0 424 761 A2. After the application of this layer, the 
whole combination film is trans-versely stretched in a ratio of from 1:7 
to 1:12, preferably from 1:8.5 to 1:9.5 and this is followed in the usual 
manner by thermofixing and corona treatment or flame pretreatment. 
It was surprising to the man of the art to find that the matt effect of 
coextruded, biaxial polypropylene films can be permanently increased if a 
polyethylene of high density is added in the given concentration ranges to 
mixtures of PP/PE block copolymers and random copolymers and to the 
modified ethylene. 
The ethylene-based co- or terpolymers used, which carry polar groups and 
are elastomer-modified, are normally used as bonding agents in an 
interlayer of composite films for joining together two adjacent polymer 
layers which cannot stick together. In such a combination, the bonding 
layer does not impair the optical properties. The values for cloudiness 
and clarity of the film are not altered. 
When used as component in a matt layer lying on the surface of the film, 
they reduce the gloss which determines inter alia the matt appearance of 
the biaxially stretched films. 
The given formulations, which are a subject matter of this Patent, produce 
a matt appearance of films which are biaxially stretched. 
In addition, the layer thickness of the matt layer can be varied over a 
wide range in biaxially orientated polypropylene films produced with a 
matt layer in accordance with the above formulation without producing any 
change in the optical properties. Matt films are obtained even with layer 
thicknesses&lt;2 .mu.m. 
The film according to the invention preferably has a total thickness of 
from 10 to 40 .mu.m. 
The matt layer M is characterised in that it preferably has a thickness of 
1 to 4 .mu.m and consists of 
25-75 % of polypropylene copolymers produced by a process of block 
copolymerisation, 
5-15% of an ethylene based co- or terpolymer modified with polar groups, 
15-40% of a random copolymer of propylene and ethylene having an ethylene 
content of from 2 to 6% by weight, 
5-20% of a high density polyethylene and 
0-2% of a natural silica. 
The core layer preferably has a thickness of 
______________________________________ 
9-39 .mu.m and consists of 
97-100% of an isiotactic polypropylene having 
a melt index of 0.5 to 8 g/10 min 
(230.degree. C./21.2 N), and 
3-0% of slip agents, antistatic agents 
and/or antiblocking agents. 
______________________________________ 
The layers S either have a preferred thickness of from 0.5 to 1.5 .mu.m and 
consist of 
______________________________________ 
97-100% of an isotactic polypropylene having 
a melt index of from 0.5 to 8 g/10 
min (230.degree. C./21.2 N) or 
a random propylene/ethylene copolymer 
containing 3 to 5% of ethylene and 
having a melt index of from 3 to 16 
g/10 min (230.degree. C./21.2 N) or 
a random propylene/ethylene butylene 
terpolymer having a comonomer content 
of &lt;12% and a melt index of from 3 
to 16 g/10 min (230.degree. C./21.2 N) and 
3-0% of slip agents, antistatic agents and/or 
antiblocking agents 
______________________________________ 
or they have a preferred thickness of from 3 to 15 .mu.m and consist of 
65-95 % of ethylene/vinyl acetate copolymers, 
10-35 % of ethylene/ethyl acrylate copolymers and 
8-30% of ethylene/acrylic acid copolymers. 
A film according to the invention is used as adhesive or thermolaminating 
film for the graphics industry or as heat sealable film for the 
manufacture of bags or of composite films. 
The following test methods and processes for the determination of the 
values and properties are employed in the examples which follow: 
The gloss is determined according to ASTM D 2457 in terms of the proportion 
of light in gloss units GE reflected at an angle of 45.degree. based on a 
black glass mirror, using 100 GE as standard. 
Cloudiness is determined according to ASTM D 1003. It is expressed in 
percent and is the ratio of diffuse light transmission to the total light 
transmission multiplied by 100.

EXAMPLES 
Substances used for the matt layers 
Polymer 1 
Block copolymer PP/PE 
MFI* (230.degree. C./21.2N) 3 g/10 min 
FNT *MFI =Melt flow index 
Melting point (DSC)* 162.degree. C. 
FNT *DSC=Differential Scanning Colorimetry 
Modulus of elasticity (DIN 53457) 1300N/mm.sup.2 
Polymer 2 
LDPE elastomer modified and acid-anhydride-modified 
MFI* (230.degree. C./21.2N) 4.3 g/10 min 
FNT *MFI =Melt flow index 
Melting point (DSC)* 104 .degree. C. 
FNT *DSC=Differential Scanning Colorimetry 
Polymer 3 
LLDPE modified with elastomeric and acid anhydride components 
MFI* (230.degree. C./21.2N) 6.2 g/10 min 
FNT *MFI =Melt flow index 
Melting point (DSC)* 125.degree. C. 
FNT *DSC=Differential Scanning Colorimetry 
Polymer 4 
P/E Random copolymer containing 1000 ppm SiO.sub.2 
MFI* (230.degree. C./21.2N) 4.7 g/10 min 
FNT *MFI =Melt flow index 
Melting point (DSC)* 140.degree. C. 
FNT *DSC=Differential Scanning Colorimetry 
Polymer 5 
High density polyethylene having a density of 0. 956 g/cm.sup.3 
MFI* (190.degree. C./50N) 1.6 g/10 min 
FNT *MFI =Melt flow index 
Melting point (DSC) 130.degree. C. 
Polymer 6 
Isotactic polypropylene containing n-heptane soluble component of 6% 
MFI* (230.degree. C./21.2N) 3.3 g/10 min 
FNT *MFI =Melt flow index 
Polymer 7 
Mixture of 65-95 % of ethylene/vinyl acetate copolymer 
10-35 % of ethylene/ethyl acrylate copolymer 
8-30 % of ethylene/acrylic acid copolymer 
__________________________________________________________________________ 
Layer 
Film Layer M 
Mixing thick- Layer 
Gloss 
Total 
structure 
of poly- 
ratio ness 
Layer K 
Layer S 
thick- 
matt 
Cloudi- 
thickness 
number 
mer % by M of poly- 
of ness S 
side 
ness 
.mu.m! 
of layers 
mixture 
weight! 
.mu.m! 
mer polymer 
.mu.m! 
GE! 
%! 
__________________________________________________________________________ 
Example 
1 15 3 1/3/5/4 
55/10/15/20 
2.3 6 6 1 7.6 79 
2 15 3 1/3/5/4 
60/10/10/20 
2.3 6 6 1 7.1 78 
3 15 3 1/2/5/4 
60/10/10/20 
2.3 6 6 1 7.0 80 
4 20 3 1/3/5/4 
65/10/5/20 
2.3 6 4 1 7.9 76 
5 22 3 1/2/5/4 
65/5/10/20 
2.5 6 7 1 8.5 72 
Comparison 
Example 
1 15 3 1/2/4 
60/10/30 
2.3 6 6 1 12.1 
60 
2 15 3 1/3/4 
40/10/50 
2.3 6 6 1 13.3 
49.1 
__________________________________________________________________________ 
Example 1 
A mixture of polymers 1/3/5/4 in a ratio of 55/10/15/20 is packed up into a 
batch. This batch forms the matt layer and is coextruded with a 
polypropylene layer which is substantially free from anti-blocking agents 
but contains slip additives and antistatic additives and with a second 
polypropylene layer containing anti-blocking agents. This is followed by 
longitudinal stretching in a ratio of 1:4.5 to 1:5.5 followed by 
transverse stretching in a ratio of from 1:9 to 1:10. Before the film is 
rolled up it is subjected to a corona pretreatment on the side which is 
not matt. The film is 15 .mu.m in thickness, the matt layer having a 
thickness of 2.3 .mu.m .+-.0.1 .mu.m, the core layer a thickness of 11.7 
.mu.m and layer S a thickness of 1.0 .mu.m. The film is used for the 
adhesive laminating of printed products. 
Example 2 
The layer structure and stretching conditions are as described in Example 
1. A mixture of polymers 1/3/5/4 in a ratio of 60/10/10/20 is used for the 
matt layer M. 
Example 3 
The same as Example 1 but using the polymer mixture 1/2/5/4 in a ratio of 
60/10/10/10 for the matt layer. 
Example 4 
A matt layer composed of a mixture of polymers 1/3/5/4 in a ratio of 
65/10/5/20 is coextruded with a polypropylene layer which is substantially 
free of additives and a sealing layer of Polymer 4 containing the usual 
additives such as anti-blocking, antistatic and slip agents. This is 
followed by longitudinal stretching in a ratio of 5:0 followed by 
transverse stretching in a ratio of from 1:9. 
Before the film is rolled up, it is subjected to a corona pretreatment on 
at least one side. The film has a total thickness of 20 .mu.m composed of 
a thickness of 2.3 .mu.m for the matt layer, 16.7 .mu.m for the 
polypropylene layer and 1 .mu.m for the sealing layer. This film is 
suitable inter alia for printing and for the manufacture of bags on 
horizontal and vertical tubular bag producing machines and both in its 
printed and unprinted form it is suitable as starting film for various 
composite films. 
Example 5 
A matt layer composed of a mixture of the polymers 1/2/5/4 in the ratio of 
65/5/10/20 is coextruded with a polypropylene layer which is substantially 
free from antiblocking agents but contains slip and antistatic additives 
and is then longitudinally stretched in a ratio of 1:5. The heat laminated 
layer is then laminated or extruded with mixture 7 as described in EP 0 
424 761 A2. 
Application of the heat laminating layer is followed by transverse 
stretching of the film in a ratio of 1:9. 
The heat laminated film thus produced has a total thickness of 22 .mu.m 
composed of 2.5 .mu.m for the matt layer, 12.5 .mu.m for the polypropylene 
layer and 7 .mu.m for the heat laminating layer. Such a film is used for 
the finishing of printed products for the graphics industry. 
Comparison Example 1 
Same as Example 1 but using the polymer mixture 1/2/4 a ratio of 60/10/30 
for the matt layer. 
Comparison Example 2 
Same as Example 1 but using the polymer mixture 1/3/4 in a ratio of 
40/10/50 for the matt layer.