U.S. patent application number 14/477345 was filed with the patent office on 2014-12-18 for method for producing polypropylene films.
The applicant listed for this patent is Treofan Germany GmbH & Co. KG. Invention is credited to Detlef BUSCH, Harald EIDEN, Christian PETERS, Josef SCHAAN.
Application Number | 20140370312 14/477345 |
Document ID | / |
Family ID | 41820385 |
Filed Date | 2014-12-18 |
United States Patent
Application |
20140370312 |
Kind Code |
A1 |
BUSCH; Detlef ; et
al. |
December 18, 2014 |
METHOD FOR PRODUCING POLYPROPYLENE FILMS
Abstract
The invention relates to a method for producing a biaxially
oriented polypropylene film including at least one layer, which is
constructed from propylene polymer B and a propylene polymer that
has been recycled once. The film is used in capacitors.
Inventors: |
BUSCH; Detlef; (Saarlouis,
DE) ; EIDEN; Harald; (Homburg, DE) ; PETERS;
Christian; (St. Ingbert, DE) ; SCHAAN; Josef;
(Lebach, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Treofan Germany GmbH & Co. KG |
Neunkirchen |
|
DE |
|
|
Family ID: |
41820385 |
Appl. No.: |
14/477345 |
Filed: |
September 4, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13132966 |
Jun 6, 2011 |
|
|
|
PCT/EP2009/008746 |
Dec 8, 2009 |
|
|
|
14477345 |
|
|
|
|
Current U.S.
Class: |
428/461 ;
428/516; 525/240 |
Current CPC
Class: |
B29C 48/21 20190201;
B29C 48/19 20190201; B29B 17/0005 20130101; B29C 48/307 20190201;
B29K 2023/12 20130101; H01G 4/18 20130101; C08J 5/18 20130101; C08J
2323/12 20130101; C08J 2423/12 20130101; B29C 48/0018 20190201;
B29K 2105/256 20130101; B29C 48/08 20190201; B29C 48/12 20190201;
B29C 48/305 20190201; B29C 48/495 20190201; B29K 2105/26 20130101;
H01B 3/441 20130101; B29L 2009/00 20130101; Y10T 428/31913
20150401; Y10T 428/31692 20150401; B29C 55/12 20130101; Y02W 30/62
20150501 |
Class at
Publication: |
428/461 ;
525/240; 428/516 |
International
Class: |
C08J 5/18 20060101
C08J005/18; H01B 3/44 20060101 H01B003/44 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 2008 |
DE |
102008061504.8 |
Claims
1.-19. (canceled)
20. A biaxially oriented polypropylene film comprising at least one
layer, wherein the film is constructed from virgin propylene
polymers and contains 0.5 to 40% by weight of at least one
polypropylene polymer that has been recycled once and no polymer
that has been recycled multiple times.
21. The film as recited in claim 20, wherein the film comprises at
least one cover layer that includes 98-100% by weight of a virgin
propylene polymer.
22. The film as recited in claim 20, wherein the film includes a
cover layer on both sides thereof, each of which includes 98-100%
by weight of a virgin propylene polymer.
23. The film as recited in claim 20, wherein the film is metallised
on one surface.
24. A process to manufacture a capacitor which comprises utilizing
the film as claimed in claim 20.
25. The film as claimed in claim 20, wherein the film is a
dielectric film.
26. The film as claimed in claim 20, wherein the film is an
electroinsulating film.
27. A capacitor which comprises the dielectric film as claim
25.
28. A biaxially oriented polypropylene film for use as a dielectric
film or electroinsulating film comprising at least one layer,
wherein the film is constructed from virgin propylene polymers and
contains 0.5 to 40% by weight of at least one polypropylene polymer
that has been recycled once and no polymer that has been recycled
multiple times.
Description
[0001] The present invention relates to a method for producing
biaxially oriented polypropylene films including at least one
layer, and use thereof.
[0002] Biaxially oriented polypropylene films (BOPP films) are used
in a wide range of packaging applications because of their good
performance characteristics. These good performance characteristics
are for example high mechanical strengths, good dimensional
stability and visual brilliance. Besides their use as packaging
films, considerable quantities of BOPP films are also used in
technical applications. These include metallisation and transfer
metallisation, lamination and use an electrical insulator in
capacitor manufacturing.
[0003] Various methods for producing biaxially oriented
polypropylene films are known from the prior art. In the "tenter
process", the BOPP film is produced by extruding, shaping in a slot
die and stretching in the longitudinal and transverse
directions.
[0004] In detail, this method is carried out in such manner that
the propylene polymers are compacted, heated and melted in an
extruder, then the molten masses corresponding to the respective
plies of the film are extruded through a flat sheet die, and the
film obtained thereby is drawn on one or more rollers to stabilise
it, the film is oriented and then thermofixed. Finally, the machine
roll produced in this way is processed to create the cut roll ready
for use by the customer.
[0005] In this process for producing biaxially oriented films, a
large amount of film trimmings is created during the process
itself, for example due to the edge strip. Edge strips are the
borders of the film that are gripped by the clips of the lateral
stretching frame but not stretched as well during transverse
stretching. The unstretched border is significantly thicker than
the film after orientation and must therefore be cut off. Depending
on the film type and machine width, the amount of material lost
with the edge strip may be as much as 25% by weight. For reasons of
economy, the edge strip must be regranulated and returned to the
original feedstock of raw material together with the rest of the
film trimmings that is created when the cut roll is prepared, in
the event of tearing or at the infeed to or outfeed from the
machine.
[0006] In some application areas requiring particularly high
quality standards of the film, it is not possible to reuse film
trimming with the original raw material feedstock in this way
again. This is true for example in the production of
electroinsulating films that are used for manufacturing capacitors.
For these applications, particular film properties such as low
dielectric loss factor, high electrical pass resistance, high DC
and AC proof voltages, and the smallest possible number of flaws
are required. One of the ways to obtain these properties is to use
exceptionally pure polypropylene with low ash and chlorine content.
In addition, the polymers must not contain any ionogenic
components. Of course, raw materials that meet these purity
requirements are significantly more expensive than conventional raw
materials for packaging.
[0007] It has been found that the high quality requirements for
such electroinsulating films are no longer met if film trimmings is
added to the ultra-pure raw material. The continuous use of trimmed
film in the production inevitably results in a certain content in
the film of material that has been recycled multiple times, since
film material that already contains some recycled material is
itself constantly returned to the recycling circuit. Material that
has already been recycled several times is also decomposed and
contaminated repeatedly in each new cycle, so the quality of the
repeatedly recycled material becomes steadily poorer. The more
cycles the respective fraction of recycled material undergoes, the
lower its proportion in the finished film becomes, but the quality
of this proportion is steadily degraded at the same rate.
[0008] The result of this in practice is that the film trimmings
generated when manufacturing electroinsulating films becomes waste
by definition, and can only be used in applications with less
stringent requirements, such as production of packaging materials
or injection moulding. As a result, the financial losses associated
with film trimmings in the production of electrofilms are
particularly significant.
[0009] Even in the field of packaging films that are produced from
relatively less expensive raw materials, the quality of the film
can be impaired by material that has been recycled multiple times,
particularly if the fraction thereof becomes too high. In this
case, therefore the proportion of material that has been recycled
multiple times in the film must be monitored, and reduced if
necessary. Thus, in this area too, there is a need for a method for
improving economy and quality in the film manufacturing
processes.
[0010] European patent EP 0 740 993 describes a method for
producing biaxially oriented polypropylene films in which the
border area is created separately from a lower-quality raw
material. The highly pure propylene polymer of the film is remelted
in a separate extruder and extruded together with this second,
lower-quality propylene polymer from a second extruder. The molten
mass of the second propylene polymer is directed along both sides
of the first propylene polymer mass in such manner that the two
molten masses are extruded together and simultaneously through the
flat sheet die, and the lower-quality propylene polymer forms the
border area of the film during the manufacturing process. After the
film has been stretched longitudinally and laterally, this border
area may be separated and used again.
[0011] In practice, it is been found that a number of disadvantages
are associated with this method. The two different polymers mingle
with one another at the boundary between the film border and the
film in such manner that is it difficult to separate the
lower-quality polymer. Either the film itself is contaminated by
the lower quality polymer in the border area, so that the quality
is not consistent over the entire width of the film, or an
excessively wide border must be cut off, thereby reducing the yield
of the method. It has also be found that after several cycles the
polymer in the border area is degraded to such an extent that is
must be replace with fresh material. This also renders the method
less financially advantageous.
[0012] Accordingly, there is still a need for suggesting a method
for producing films from high-quality, for example particularly
pure, polypropylene, that avoids the disadvantages described in the
preceding. The method is intended particularly to reduce the
financial losses associated with film trimmings that cannot be
reused.
[0013] This object is solved with a method for producing a
biaxially oriented polypropylene film, consisting of at least one
layer, and in which a polypropylene polymer B is heated and melted
in a first extruder, and the molten mass of the propylene polymer B
is extruded through a flat sheet die, and the molten mass exiting
the flat sheet die is drawn on one or more rollers to and
solidified to form the precursor film, and this precursor film is
stretched longitudinally and laterally, characterized in that
[0014] A. a propylene polymer or a propylene polymer mixture A with
no recycled content A is heated and melted in a second extruder,
and [0015] B. the molten mass of the propylene polymer or propylene
polymer mixture A is directed along both sides of molten propylene
polymer mass B in such manner that all of the molten masses are
extruded through the flat sheet die together and at the same time,
and the propylene polymer or propylene polymer mixture A with no
recycled content forms the border area of the film during the
manufacturing process, and [0016] C. the border areas consisting of
the propylene polymer or propylene polymer mixture A are cut away
after the film has been stretched longitudinally and laterally, and
[0017] D. this material cut off from the border area is mixed with
the polypropylene polymer B and this mixture is melted and extruded
to form the precursor film in the first extruder.
[0018] The method according to the invention ensures that all of
the film trimming from the border area is reusable, but the film
contains no material that has passed through the treatment process
more than once. This, the final product consists entirely of virgin
material and material that has undergone not more than one
recycling step.
[0019] For the purposes of the present invention, virgin material
or virgin polymer is understood to mean a polymer or polymer
mixture that has not previously been used in a film manufacturing
process and has not undergone a melting process with subsequent
solidification of the molten mass for film production. With regard
to a polymer mixture, the same applies for all components of the
mixture. In this sense, both polypropylene polymer B and
polypropylene polymer A are virgin polymers. Both polymer A and
polymer B may also be mixtures of various virgin propylene
polymers.
[0020] For the purposes of the present invention, recycled material
is a polymer or polymer mixture that has already been used as
defined in a film manufacturing process, and has been melted and
resolidified during film production and possibly in a subsequent
treatment process as well.
[0021] For the purposes of the present invention, material that has
been recycled multiple times is recycled material that has been
melted and solidified more than twice in total during film
production and treatment.
[0022] For the purposes of the present invention, material that has
been recycled once is recycled material that has been melted and
solidified either only once, during film production, or twice the
second time in a subsequent treatment step. When material that has
been recycled once is used to manufacture a film, this portion of
the recycled material is melted and solidified again, of
course.
[0023] In this way, a film that is manufactured according to the
method of the invention consists primarily of virgin polymer B,
which has been melted and resolidified only once, during the actual
film production process. Material that has been recycled once also
contains fractions from a polymer or polymer mixture A that has
been melted and solidified two or three times, that is to say a
first time when it passed through the film manufacturing process as
the border strip of the film, and a second time by mixing with
polymer B to produce the film product, and possibly also a third
time during processing of the border strip trimming, to turn it
into granulate for example. Accordingly, polymers A and B differ in
that polymer B is melted and solidified in the film once, and
polymer A is melted in the film twice or three times. In general,
the film contains from 0.5 to 60% by weight of material that has
been recycled once from polymer A, preferably 1 to 50% by weight,
particularly 5 to 40% by weight, and correspondingly 40 to 99.95%
by weight and preferably 50 to 99% by weight of polymer B, relative
to the total weight of the film in each case. In all cases, it is
possible that either or both of polymer B and the border strip
polymer A also consist of a mixture of various polymers. According
to the invention, all fractions of such a polymer mixture B are
virgin material and all fractions of a polymer mixture A, which are
used to manufacture the film, have been recycled no more than
once.
[0024] In all cases, in the method according to the invention
different polymers may used as propylene polymer B for the film and
as propylene polymer A for the border area, similarly to the method
described in EP 0 740 993. However, unlike the method described in
EP 0 740 993, polymer A for the border area is to be selected such
that it generally satisfies the requirements for film manufacture
as well, since it is to be used in the film itself after it has
been used as border material. Consequently, in the method according
to the invention it is generally preferred if the same polymers, or
at least polymers of comparable quality, are used in the border
area and in the film. According to the invention, the trimmed
material from the border area is not used for border extrusion
again, instead it is mixed with the film polymer B and used
together to this to produce the film, to ensure that the film
itself only contains virgin raw material and material that has been
recycled only once.
[0025] Surprisingly, the quality of the film, and particularly its
electrical properties, are not impaired by the addition of the
material that has been recycled once. According to the prior art,
no recycled material is used in electroinsulating films because the
electrical properties are impaired by the material that has been
recycled multiple times. In packaging films, excessively high
quantities of material that has been recycled multiple times can
degrade the mechanical properties or transparency. It is assumed
that these impairments are caused by an excessively high proportion
of material that has been recycled multiple times, although their
proportion becomes progressively smaller as the number of recycling
steps increases. Surprisingly, when material that has been recycled
only once is added, only minor impairments are caused in the film
properties, or none at. The slight impairments of film properties
caused by material that has been recycled only once are of such an
order that they may be compensated by reducing the proportion of
material that has been recycled once.
[0026] In the following, suitable polymers both for polymer A of
the border area and for polymer B of the film will be described.
Polymers A and B may, but do not necessarily have to have identical
properties. They are preferably the same polymers. Thus the
designation as polymer A and polymer B is not indicative of a
different composition or different structure of the polymers, but
rather of the different use of the polymers designated as such, on
the one hand in the border area (polymer A) and on the other in the
films (polymer B).
[0027] In general, polymers A/B have a residual ash content less
than or equal to 70 ppm, preferably .ltoreq.50 ppm, particularly
.ltoreq.40 ppm, and a chlorine content of .ltoreq.50 ppm,
preferably .ltoreq.20 ppm.
[0028] In general, polymers A/B contain 90 to 100% by weight,
preferably 95 to 100% by weight, particularly 98 to 100% by weight
propylene units relative to the weight of the polymer. In general,
polymers A/B have a melting point of 150.degree. C. or higher,
preferably 155 to 170.degree. C., and a melt flow index of 0.5 g/10
min to 10 g/10 min, preferably 0.8 g/10 min to 5 g/10 min, measured
at 230.degree. C., and a force of 21.6 N (DIN 53 735).
[0029] Particularly suitable polymers NB have an average molecular
weight Mw in the range from 150,000 to 400,000, preferably from
180,000 to 350,000. The molecular weight distribution may vary
within wide limits, Mw/Mn is generally from 2 to 15, preferably
from 2 to 6, particularly from 3 to 6.
[0030] Of the polymers A/B described above, isotactic propylene
homopolymer with an n-heptane soluble fraction of 1 to 15% by
weight, preferably 1 to 10% by weight, and a chain isotaxy index of
the n-heptane insoluble fraction of .gtoreq.85%, preferably
.gtoreq.90%, are particularly preferable. Copolymers of ethylene
and propylene having an ethylene content of 10% by weight or less,
copolymers of propylene with C4-C6 olefins having an olefin content
of 10% by weight or less, terpolymers of propylene, ethylene and
butylene having an ethylene content of 10% by weight or less and a
butylene content of 15% by weight or less are also suitable. The
weight percentages cited are relative to the respective propylene
polymer.
[0031] In order to improve certain properties of the polypropylene
film according to the invention, stabilisers and/or neutralisers,
and possibly nucleating agents as well are generally added to
polymers A/B. With regard to the desired electrical properties of
the film, in a preferred embodiment no antistatic agents and no
lubricants should be added, because additives have a negative
effect on the electrical properties of the film. All quantities
indicated in percent by weight (% by weight) in the following
description are relative to the layer or layers to which the
additive may be added.
[0032] The usual compounds with stabilising effects for polymers of
polyethylene, propylene and other alpha-olefins may be used as
stabiliser. The quantity in which these are added is between 0.05
and 2% by weight. Phenolic stabilisers, alkaline/alkaline earth
stearates and/or alkaline/alkaline earth carbonates are
particularly suitable. Phenolic stabilisers are preferred in a
quantity from 0.1 to 0.6% by weight, particularly 0.15 to 0.3% by
weight, and having a molar mass of more than 500 g/mol.
Pentaerytrhritol tetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propionate or
1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene
are particularly advantageous.
[0033] Neutralisation agents are preferably calcium stearate and/or
calcium carbonate having an average particle size not exceeding 0.7
.mu.m, an absolute particle size smaller than 10 .mu.m and a
specific surface area of at least 40 m2/g. In general, the
neutralisation agent is added in a quantity of 0.02 to 0.5% by
weight.
[0034] Nucleation means may be organic substances, preferably
dibenzylidene sorbitol or chemically modified derivatives of
dibenzylidene sorbitol or
sodium-bis-(4-tert.-butylphenyl)phosphate. Other nucleation agents
that may be used are for example metal salts of benzoic acid,
preferably sodium benzoate, and quinacridone and quinacridone
derivatives. Inorganic nucleation agents such as talcum, silicon
dioxide or bentonite are also suitable. In this case, it is most
important that the nucleation agent is extremely finely
distributed, that is to say the average particle size is not more
than 1 .mu.m, preferably not more than 0.7 .mu.m.
[0035] If polymers A and B are not identical, the melting points of
the two polymers should be approximately the same, that is to say
they should advantageously not differ by more than 15.degree. C.,
preferably not more than 10.degree. C., or they should be
approximately the same. Moreover, if the melt flow indices (MFI) of
the two raw materials are different, they must generally be matched
with one another so that the edge strips and the film are firmly
bonded with one another. The MFI of propylene polymer A should
preferably have a value not more than three times greater than the
MFI of propylene polymer B. If necessary, the melt flow indices of
the two polymers may be equal, or the MFI of propylene polymer A
may be slightly smaller (10%) than that of propylene polymer B.
[0036] It is possible that the thickness of the film edge may be
deliberately varied or adjusted even with a constant die setting by
means of the MFI of polymer A, without changing the thickness of
the film itself. The larger the MFI of polymer A relative to the
MFI of polymer B, the thicker the border area becomes and vice
versa. In this way, the thickness of the edge strip may be
optimised independently of the film that is to be manufactured.
This is particularly advantageous when producing very thing films.
According to conventional methods, the die must be opened very wide
in the border area to enable production of a sufficiently thick
edge strip. In this event, the there is a danger that the die lips
will be bent irreparably. According to the method of the invention,
such extreme die settings are no longer necessary for manufacturing
very thin films.
[0037] According to the method of the invention, single-layer or
multi-layer films may be produced. Multi-layer polypropylene films
include the film described in the preceding, or the layer of these
single-layer embodiments as the base layer, and at least one
covering layer, possible one on either side, containing in general
75 to 100% by weight, preferably 90 to 99.5% by weight relative to
the weight of the covering layer in each case. polymers of olefins
having 2 to 10 carbon atoms and optionally also usual additives,
each in effective quantities, provided they do not negatively
affect the electrical or other desired properties.
[0038] Suitable olefin polymers for the cover layer(s) are for
example homopolymers, co- or terpolymers created from ethylene,
propylene or butylene units, wherein terpolymers contain three
different monomers. The composition of the co- or terpolymers from
the respective monomers may vary within broad limits. In general,
the co- and/or terpolymers contain more than 50% by weight
propylene units, that is to say they are propylene co- and/or
propylene terpolymers with ethylene and/or butylene units as
comonomers. Copolymers generally include at least 60-99% by weight,
preferably 65 to 97% by weight propylene and not more than 1-40% by
weight, preferably 3 to 35% by weight ethylene or butylene as
comonomers. Terpolymers generally include 65 to 96% by weight,
preferably 72 to 93% by weight propylene, and 3 to 34% by weight,
preferably 5 to 26% by weight ethylene and 1 to 10% by weight,
preferably 2 to 8% by weight butylene. The melt index of the co-
and/or terpolymers is generally from 0.1 to 20 g/10 min
(190.degree. C., 21.6N), preferably from 0.1 to 15 g/10 min. The
melting point may be in a range from 70 to 150.degree. C.,
preferably from 100 to 140.degree. C.
[0039] The co- and terpolymers described in the preceding may be
mixed with each other. In this case, the proportions of co- and
terpolymers may be varied at will.
[0040] In a preferred embodiment, propylene homopolymer is used in
the covering layer(s) instead of the name co- and/or terpolymers.
Suitable propylene homopolymers are those that were described
individually as virgin propylene homopolymers B of the film in the
preceding.
[0041] For the use as electroinsulating films, embodiments are
particularly preferred that include cover layers on one or both
sides, which in turn are constructed from virgin propylene polymer,
that is to say have a content of this virgin propylene polymer in
an amount from 90-100% by weight, preferably 98-<100% by weight.
Suitable virgin propylene polymers have been described in the
preceding as polymers A/B.
[0042] It was found that these embodiments offer additional
advantages when they are used in capacitors. This is attributed to
the fact that the base layer, which contains material that has been
recycled once as well as polymer B, is covered by at least one
covering layer without an recycled material, so that any possible
negative effects of the recycled material component are reduced.
Accordingly, it is particularly preferred to cover the base layer
containing recycled material on both sides with cover layers that
do not contain any recycled material.
[0043] Multiple-layer embodiments of films that are manufactured
according to the method of the invention include a base layer and
at least the cover layer described in the preceding. The base layer
may also have cover layers on both sides and possibly additional
intermediate layers. For the purposes of the present invention, the
base layer is the layer that constitutes from more than 50 to 100%,
preferably 70 to 95% of the total film thickness. The cover layer
is the layer that forms the outer layer of the film.
[0044] The total thickness of the films that may be produced
according to the method of the invention may be varied within wide
limits, and is adapted to its intended use. The preferred
embodiments of the film have total thicknesses from 2 to 100 .mu.m,
and 2 to 50 .mu.m, particularly 2 to 20 .mu.m are preferred. The
thickness of any intermediate layer(s) present is from 0.5 to 15
.mu.m. The thickness of the cover layer(s) is preferably in the
range from 0.1 to 10 .mu.m, particularly 0.2 to 5 .mu.m, and cover
layers that are applied on either side may have the same or
different thicknesses and compositions. The thickness of the base
layer is determined by the difference between the total thickness
of the film and the thickness of the applied cover and intermediate
layer(s), and may thus vary in similar manner to the total
thickness.
[0045] In the course of the method according to the invention,
propylene polymers B and material recycled once from polymer A are
mixed, compacted, heated and melted (molten mass 1) in a first,
main extruder (EXTR. 1). Propylene polymer A (only virgin material)
is also compacted, heated and melted (molten mass 2) in a second
extruder (EXTR. 2). No material that has already been used in the
manufacture of a film is added to propylene polymer A (called
"virgin" for the purposes of the invention). Molten mass 2 of the
virgin propylene polymer A is directed towards the two sides of
molten mass 1 consisting of polymer B and material that has been
recycled once, such that the two molten masses 1 and 2 are extruded
together and simultaneously through the flat sheet die, and
propylene polymer A forms the border area of the film during the
manufacturing process. Guidance of polymer molten mass 2 towards
the edge of molten mass 1 may be effected, as shown in FIG. 3,
using a coextrusion adapter that has been rotated through
90.degree.. Of course, when there is no recycled material present
at the start of the manufacturing process, molten mass 1 will
consist only of polymer B (with recycled material), until enough
recycled material from polymer A has been produced to feed it back
into the process.
[0046] FIGS. 3 and 3A represent the use of coextrusion adapter 6 in
an arrangement according to the invention. The process of
compacting, heating and melting the polymers and feeding is similar
for both molten mass flows 1 and 2. Molten mass flows 1 and 2 are
arranged beside one another as shown (16). The adjacent molten
masses are extruded in slot die 8 to create film (10), whose border
areas (12) are produced from polymer A (from the molten mass flow
from extruder 2). The actual film (14) consists of polymer B (from
the molten mass flow from extruder 1).
[0047] A mono nozzle with which molten mass A is injected laterally
is also suitable for the method according to the invention. The
basic construction of such a mono nozzle is shown in FIGS. 4A and
4B. Slot die 8 has one aperture 20 each to the left and right of
main channel 18. Molten mass B is forced through main channel 18 in
the central part of the die. Molten mass A flows through apertures
20 into the two border areas of the die. In this way, molten masses
1 and 2 are extruded side by side to form a film whose border areas
consist of polymer A.
[0048] The film extruded in this way is drawn on one or more
rollers to solidify it. It has also proven particularly
advantageous if the draw-off roller(s) by which the extruded film
is solidified are maintained at a temperature of at least
70.degree. C., preferably 80 to 120.degree. C.
[0049] The prefilm obtained in this way is stretched lengthwise and
transversely to the extrusion direction, causing the molecule
chains to be oriented biaxially. This biaxial orientation is
carried out consecutively, and stretching preferably performed
lengthwise first (in the direction of the machine) and then
transversely (perpendicularly to the direction of the machine). In
the lengthwise direction, the material is preferably stretched by a
factor of 4:1 to 9:1, particularly 5:1 to 8.5:1, and transversely
preferably by a factor of 6:1 to 11:1. Lengthwise stretching will
ideally be carried out with the aid of two rollers running at
different speeds in keeping with the intended stretching ratio, and
transverse stretching with the aid of a corresponding clip frame.
The clips clasp the border area of the film so that essentially
only the mixture of polymer B and material that has been recycled
once from polymer A is stretched into a thin film (14) and the
borders (12) of polymer A remain unstretched and thick. This is
shown diagrammatically in FIG. 1.
[0050] The temperatures at which lengthwise and transverse
stretching are carried out may vary within a wide range and are
determined by the respective composition of the layers and the
desired properties of the film. In general, lengthwise stretching
is carried out at 80 to 160.degree. C., preferably 100 to
160.degree. C., and transverse stretching at 120 to 170.degree. C.,
preferably 130 to 160.degree. C.
[0051] After biaxial stretching, the film is immediately
thermofixed (annealed), wherein the film is maintained at a
temperature of 100 to 160.degree. C., preferably 110 to 130.degree.
C., for a period of 0.1 to 10 seconds.
[0052] As indicated previously, after biaxial stretching one or
both surfaces of the film is/are subjected to one of the known
corona, flame or plasma treatment processes. The treatment
intensities are within the normal parameters, which are 35 to 50
mN/m, preferably 36 to 45 mN/m.
[0053] For the alternative corona treatment, the film is drawn
between two electrodes that serve as guide elements, wherein the
voltage between the electrodes, usually AC voltage is so high
(about 10,000 V and 10,000 Hz), that corona discharges can take
place. The corona discharge causes the air above the film surface
to ionise and react with the molecules in the film surface, so that
polar pockets are created in the essentially apolar polymer
matrix.
[0054] After the optional surface treatment or thermofixing, the
film is trimmed with standard cutting devices, and the film itself
is rolled up on a known winding device. In general, the width of
the two borders to be trimmed is up to 300 mm, preferably 100 to
200 mm, and consists of at least 90% by weight, preferably 95-100%
by weight, and especially 99-100% by weight of propylene polymer A.
When the edge strip is separated, the width of the trimmed border
should advantageously be chosen such that as far as possible all of
the material separated as edge strip consists in a proportion of
almost 100% by weight, that is to say solely of virgin polymer A.
The advantage of the method according to the invention also
consists in that it is not necessary to cut away the edge strip so
deeply that the film contains no edge strip polymer A at all, since
polymer A is now of sufficiently high film quality and does not
form any areas of lower quality. In contrast, in the method
according to EP 0 740 993 it is imperative to trim the edge strip
away at such a point that ensures all content of polymer A is
removed and the film contains no polymer A whatsoever. This then
results in edge strip trimming that consists of a mixture of
polymer A and small quantities of polymer B. At all events, the
method according to EP 0 740 993 requires trimming a relatively
wide edge strip, whereas in the method according to the present
invention the width of the edge strip to be trimmed may be kept
narrow. Of course this applies particularly if identical raw
materials are used as both polymer A and B, as is the case in a
preferred embodiment.
[0055] In standard production methods, the film border has a
thickness of up to 200 .mu.m, generally from 20 to 100 .mu.m,
preferably 20 to 50 .mu.m. Depending on the circumstances, border
thicknesses greater or less than this may also be suitable.
[0056] According to the invention, the cut (trimmed) border areas
are chopped, granulated if necessary, mixed with polymer B so that
it may be reused in the film as recycled material. The method
according to the invention ensures that propylene polymer A in the
finished film has undergone the extrusion process not more than
three times, preferably only twice, first as edge strip material
and then as an admixture to the actual raw material for the film. A
third extrusion step may be added if the cut edge strip material is
regranulated before mixing with polymer B. Since the edge strip of
the film is continuously run using exclusively virgin polymer in
the manufacturing process, it is not possible to produce any
material that has passed through an extrusion process multiple
times, that is to say more than two or three times. Surprisingly,
neither the electrical nor any other properties of the film are
affected negatively by the fraction of polymer that has been
recycled once.
[0057] The film that is manufactured according this inventive
method lends is therefore ideally suitable for use as a dielectric
in capacitors. If necessary, the film may be metallised beforehand
using methods known from the related art. It has been found that
the fractions of recycled materials impair the performance
characteristics of the capacitors only to a very limited degree, if
at all. It is thus possible to achieve significant cost advantages
without loss of quality.
[0058] The following measuring methods were used to characterize
the raw materials and films:
[0059] Melt Flow Index:
[0060] The melt flow index was measured under a load of 21.6 N and
at 230.degree. C. in accordance with the specifications of DIN 53
735.
[0061] Melting Point:
[0062] DSC measurement, maximum on the melting curve, heating rate
20.degree. C./min.
[0063] E Modulus
[0064] The E modulus will be determined at least 10 days after
production in accordance with EN ISO 521-1 on a specimen having a
size of 15*100 mm.sup.2.
[0065] Shrinkage:
[0066] Longitudinal and transverse shrinkage values are relative to
the linear dimension of the film expansion in each direction
(lengthwise L.sub.0 and transverse Q.sub.0) before the shrinking
process. The lengthwise direction is the direction of the machine,
and the transverse direction is correspondingly defined as the
direction perpendicular to the direction of the machine. The
specimen of 10*10 cm.sup.2 will be shrunk in a convection oven at
the respective temperature (from 100 to 140.degree. C.) for a
period of 15 min. The remaining linear dimensions (L.sub.1 and
Q.sub.1) of the specimen will be determined again for the
lengthwise and transverse directions. The difference between the
determined linear dimensions and the original dimensions L.sub.0
and Q.sub.0 is then expressed as shrinkage in a percentage of the
original length times 100.
L a ngsschrumpf L s [ % ] = L 0 - L 1 L 0 * 100 [ % ] ##EQU00001##
Querschrumpf Q s [ % ] = Q 0 - Q 1 Q 0 * 100 [ % ]
##EQU00001.2##
[0067] This method for determining lengthwise and transverse
shrinkage corresponds to DIN 40634.
[0068] Dielectric Loss Factor:
[0069] The dielectric loss factor (tan .alpha.) is calculated
according to VDE 0303, Part 4. Both sides of the film specimens are
coated with aluminium vapour in a vacuum coating plant before the
measurement. The dimensions of the measurement area F (=coated
area) vary according to the film thickness d:
for a film thickness d.ltoreq.10 .mu.m, an area of 1 cm.sup.2
for a film thickness d>10 .mu.m, an area of 5 cm.sup.2
[0070] A double determination is carried out on each specimen to be
tested, and the average is calculated therefrom. The specimens are
placed in a drying cabinet. The lower electrode plate is made of
brass. The upper electrode is cylindrical and is also made of
brass. The test voltage is 1V. One measurement is taken at each of
three frequencies, 0.1 KHz, 1 KHz and 10 KHz.
[0071] Residual Ash Content:
[0072] In order to measure the residual ash content, the fraction
of non-combustible mineral filler substances is determined
quantitatively. The residual ash content (annealing loss) is
calculated from the original weight of the specimen and the ash
content. The measurement result is indicated in ppm. A
representative random sample of about 1 kg is remove from the
material to be tested (granulate, regenerate, etc.). The material
must be clean and completely dry; it may be necessary to pre-dry
the specimen in a convection warming cupboard at about 80.degree.
C. Three empty porcelain crucibles are annealed in the crucible
furnace for at least 1 h at a temperature of 650.degree. C., and
after cooling to room temperature in the desiccator are weighed
with an accuracy of 0.1 mg. Annealing is repeated until a constant
weight is achieved between two consecutive weighings. Then, 50 g
(+/-0.1 g) material is weighed into each crucible, which are placed
in the muffle kiln, which has been preheated to 650.degree. C. The
temperature in the kiln is now raised to 1,000.degree. C. and kept
constant at this temperature for at least 1 h. After the crucibles
have been cooled in the desiccator, they are weighed again with an
accuracy of 0.1 mg. The ash content is expressed with the
measurement unit ppm (parts per million)=mg/m.sup.3. All three
crucibles are evaluated according to the following formula, and the
two with values closest to one another are combined to yield an
average:
ppm=Final weight (g)/Initial weight (g).times.1,000,000
[0073] Chlorine Content:
[0074] The chlorine content in polyolefins is measured
quantitatively by X-ray Fluorescence Spectroscopy (XFS) in
accordance with DIN 51 001, Part 1. A tablet of made from compacted
granulate/powder, and this is measured against a calibration curve
using XFS. The calibration curve was compiled on the basis of 10
calibration specimens in which the chlorine content was determined
in an unrelated method (wet process).
[0075] Determination of Molecular Weight:
[0076] In order to determine the average molecular weight Mw, three
detector gel permeation chromatography is used. The substance is
dissolved in an eluent such as THF and injected onto a separation
column. The separation column is 90 cm long and filled with a
porous carrier material with a pore size of 5 .mu.m. Detection is
carried out using UV absorption spectroscopy at various
wavelengths, also on the basis of the refractive index and light
scattering capability of the fractions. Calibration is performed
via a standard composition having known molecular weight. Molecular
weights are able to be assigned by comparing the UV absorption of
the standard substance with the absorption of the specimen (DIN 55
672 Part 1).
[0077] The invention will now be explained in greater detail with
reference to exemplary embodiments:
EXAMPLE 1
[0078] A transparent film having thickness of 6 .mu.m and an
untrimmed width of 4940 mm was produced by extrusion and subsequent
progressive orientation in the longitudinal and transverse
directions. The film thus produced consisted of a single layer. At
the start of production, the adjacent molten masses "Trimming
strip-Film-Trimming strip" were drawn out of the same raw material.
For common extrusion and arrangement of the separate melt flows,
the adapter equipment was used, and had been rotated through
90.degree. with respect to the usual arrangement for multilayer
coextrusion
[0079] B-Original Polymer for Film: [0080] .about.100% by weight
highly isotactic polypropylene manufactured by Borealis (brand name
HB 300 BF) with a melting point of 165.degree. C. and a melt flow
index of 3.5 g/10 min at 230.degree. C. and 2.16 N, having a
residual ash content of about 20 ppm and a chlorine content of
<1 ppm [0081] 0.45% by weight Irganox 1010 phenolic stabiliser
[0082] 0.0075% by weight Ca stearate as neutralising agent
[0083] A-Trim Strip Polymer: [0084] .about.100% by weight highly
isotactic polypropylene manufactured by Borealis (brand name HB 300
BF) with a melting point of 165.degree. C. and a melt flow index of
3.5 g/10 min at 230.degree. C. and 2.16 N, having a residual ash
content of about 20 ppm and a chlorine content of <1 ppm [0085]
0.45% by weight Irganox 1010 phenolic stabiliser [0086] 0.0075% by
weight Ca stearate as neutralising agent
[0087] After startup, a 10 cm wide trimming strip was separated on
both sides in the outfeed area and before the film was wound up,
which strip was then reduced in a chopper. This chopped film
material was then transported via conveyor means to the extruder
together with raw film material B, with which it was mixed and
melted together with the original raw material. The border area was
still running continuously with virgin original raw material.
[0088] The production conditions in the various process steps
were:
TABLE-US-00001 Extrusion: Temperatures 250.degree. C. Temperature
of drawing roller: 97.degree. C. Lengthwise stretching: Lengthwise
stretching temperature 150.degree. C. Lengthwise stretching ratio:
5.5 Transverse stretching: Temperature: 160.degree. C. Transverse
stretching ratio: 9.5 Annealing: Temperature: 140.degree. C.
Convergence: 15%
[0089] The transverse stretching ratio is an actual value. This
actual value is calculated from the final film width less twice the
trim strip width, divided by the width of the longitudinally
stretched film, which is also reduced by twice the trim strip
width.
[0090] In this way, a film was produced that contained
approximately 22% by weight of raw material that had been recycled
once
EXAMPLES 2a TO 2c
[0091] Compared with example 1, the chopped film material was
treated by one-time melting and cooling to produce a regranulate in
a separate processing step. This regranulate was then processed
together with film raw material B in similar manner. In this way,
more films were produced containing 5%, 10%, 30% raw material that
had been recycled once.
Comparison Example 1
[0092] Compared with example 1, the border area was not run with a
separate molten mass flow, the film and border area were drawn from
one extruder. As was described in example 1, the border area was
trimmed, chopped, and reused in the production of the film. In this
way, a film was produced containing approximately 22% of raw
material that had been recycled multiple times.
Comparison Example 2
[0093] Compared with example 1, the border area was not run with a
separate molten mass flow, the film and border area were drawn from
one extruder. The border area was trimmed and disposed of as waste,
that is to say the film was produced using only original raw
material and contained no recycled raw material regardless of the
number of times it had been recycled.
EXAMPLE 3
[0094] A film was produced as described in example 1. Unlike
example 1, a three-layer film was co-extruded having a cover layer
on both surfaces of the base layer. The base layer corresponded to
the film of example 1. Both additional cover layers were
constructed from 100% by weight propylene polymer B as described in
example 1. The thickness of the base layer was about 5 .mu.m. The
thickness of each cover layer was about. 0.5 .mu.m, so that the
total thickness of the film as in example 1 was 6 .mu.m. The rest
of the composition and the process conditions were unchanged from
example 1. In this way, a three-layer film was produced containing
approximately 26% by weight of polymer that had been recycled once
in the base layer (corresponding to 22% by weight relative to the
film as a whole).
EXAMPLE 4
[0095] A film was produced as described in example 2b. Unlike
example 2b, a three-layer film was co-extruded comprising a cover
layer on both surfaces of the base layer. The base layer
corresponded to the film of example 2b. Both additional cover
layers were constructed from 100% by weight propylene polymer B as
described in example 2b. The thickness of the base layer was about
5 .mu.m. The thickness of each cover layer was about. 0.5 .mu.m, so
that the total thickness of the film as in example 2b was 6 .mu.m.
The rest of the composition and the process conditions were
unchanged from example 2b. In this way, a three-layer film was
produced containing approximately 12% by weight of polymer that had
been recycled once in the base layer (corresponding to 10% by
weight relative to the film as a whole).
TABLE-US-00002 Breakdown voltage E modulus AC/DC MD/TD Shrinkage
Flaws as number Example [KV/mm] [N/mm2] MD/TD [%] per sq.m. at 300
V 1 419/813 2820/4700 3.0/0.60 0 2a 410/795 2825/4710 3.0/0.62 0 2b
425/780 2798/4680 3.2/0.65 0 2c 428/755 2752/4580 3.2/0.65 1 VB1
380/650 2500/4300 3.4/0.66 5 VB2 420/815 2910/4850 3.1/0.62 0 3
420/813 2870/4750 3.0/0.60 0 4 418/805 2875/4720 3.2/0.60 0
* * * * *