Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PREPARING A WEB OF FILM PRIOR
TO WINDING IT INTO A WOUND ROLL
The present invention relates to a process for preparing a web
of thermoplastic film for a subsequent winding operation in which a llne
is embossed in a flat web of material. The line serves as a spacer be-
tween the individual layers of the wound roll and thus ensures that the
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good planarity of the web of material produced during the manufacture
thereof is retalned in the wound roll even over a prolonged period of
time.
It already has been proposed to provlde the edge zones of D
web of film with a trace, for example a so-called "cording", which is
produced by means of a drum or roller covered with spikes and counter-
acting with a counter-drum or counter-roller. After its manufacture and
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after leaving the machine, the cold web is passed through this pair of
` rollers and the rollers are pressed upon one another wlth a force which
ls sufficient to cause the film material to be displaced by the spikes
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forced into it so that small mounds are formed. Depending upon the
thickness of the film, the shape of the spikes and the pressure applied,
these mounds pro~ect somewhat from the plane of the material and thus
form detiberately produced thicker areas which, by being constantly
superimposed, determine the diameter of the wound roll and cause the
individual layers to be slightly spaced from each other in the area Iying
between the embossed lines, which are preferably arranged along the
edges. In this manner, the surfaces of the films are generally pro-
tected, because over most of the area the surfaces of the indivldual
Iayers are no longer in contact with each other.
This treatment of the web of material, which is part of the
prior art, has the disadvantage that, by the individually slight, but as
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a whole distinctly noticeable displacement of the film material, it pro-
duces a certain expanding effect, slmllar to the chasing of a copper
sheet lnto an arched form, which causes a wavy distortlon of the treat-
ed edges. Thls devlation from the planarity of the fllm is completety
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unacceptable in the case of high quality films, for example films whlch
are to be used as supporting materials for photographic and reprographic
' products or for drawing materials, because the quallty of the finished
' products is substantially determlned by the planarity of the material.
Further, it is known from Belgian Patent No. 764,289 to produce
a definite distance between the individual windings of a roll by applying
a foreign substance, for example a wax mixture, to the desired area
~S~ prior to the winding operation, thus producing an area of a controlled
y thickness. This process does not result in the adverse distortion of the ;
film web, but it involves the risk that, when the web is unwound after-
wards for further processing, particles of the foreign substance may
split-off and get into the useful area of the film, which may give rise
to coating defects and the like . Therefore, wound rolls of this type must
be carefully observed during their further processing. Of course, the
application of the wax mixture increases the manufacturing costs of
ti-ese rolls .
Further, it is known from French Patent No. 2,110,951, to
prevent the blocking of webs of materials by winding strips or threads
'; having a different coefficient of friction between the layers of the mater-
~al during the winding-up process. This process which is not aimed at
maintaining the planarity of a wound web of film, has the disadvantage
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that it involves considerabte additional expenses for the strips and for
the winding-off and winding-up installations, that these installations
must be constantly observed because the rolls are not replaced at the
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same intervals as ordinary rolls, and that suitable strips for insertion
must be constantly provided.
i~ According to a last group of known processes, desired areas of
thicker cross-section in the web are prepared by appropriately preforming
the base film at the die orifice. This process has also some distinct
disadvantages. The somewhat thicker bands within the web of material must
pass with the web through different stretching processes during further
processing of the material, and it was found that they adversely affect the
uniform temperature distribution in the material, that the bands are prone
to irregular lateral migration and thus are not superimposed when the web
; is subsequently wound up, and that, due to several physical causes, the
stretching processes create a dispnoportionate amplification effect of
positive and negative deviations from the normal profile which are not exact-
ly controllable. With these processes, a specific arrangement of the bands
in the web cannot be repeated, so that it is impossible to produce wound
rolls of uniform construction by these processes.
It is the object of the present invention to provide a technically
relatively simple process by which a web of flat film is provided with
` embossed lines in a manner such that the good planarity of the film web
` 20 produced during its manufacture is not impaired, but is retained even if
the web is stored in the wound state over a relatively long period of time.
This object is achieved by a process for preparing an extruded web
-~- of flat thermoplastic film for a subsequent winding operation, by providing
it with embossed lines along the edges or very close to the edges of the web
in which process the web of film is subjected to tensional forces acting on
the edges of the web in the direction of the plane of the web without
stretching the web, while, at the same time, the embossed lines are applied
to the tensioned part of the film web and the embossed and tensioned film is
subjected to a thermal treatment.
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For this purpose, the edges of the film web may be held, for
example, in revolving clip chains, by belts, or similar known means. The
tension may be constant, or it may be increased, for example, during the
application of the embossed lines and/or during thermal treatment, e.g.,
by using a diverging tenter. Since a certain longitudinal tension is always
present due to the transportation of the film web, although it may be main-
tained at a value near zero, the requirement for an all-directional tension
of the film web during the process steps is fulfilled. The term "thermal
treatment" includes heating processes as well as cooling processes, as shown
below with reference to a specific embodiment of the present process.
The advantages of the present process are many. By the exact
transverse tension of the web of material produced, e.g., by clamps gripping
the edges of the web, and the well-balanced longitudinal tension within the
treated areas, the web is in an excellent state of tension and planarity
when the embossed lines are applied. As a further advantage, this absolute
planarity of the web of material is retained during the further transport
of the material, and during the subsequent thermal treatment of the whole
web. The application of the embossed lines and the subsequent thermal treat- -
ment according to the invention practically force the web to assume a
position of optimum planarity. At the same time, the lines applied according
to the invention are fixed and the rims of the tiny mounds formed by emboss-
ing are subjected to a thermal treatment without distortion. .
When the web of material is wound up, the embossed lines applied
to it according to the invention become effective in that the
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' superimposed lines form an annular, or, when viewed as a whole,
disc-shaped construction of good supporting capacity within the wind-
ings of the roll and create the desired air film between the individual
layers, which prevents the layers from Iying upon one another and
thus protects them from being scratched.
Accordlng to a particularly advantageous embodiment of the
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;i ~ invention, the embossed lines are applied while the web of film is in
the ductite state. The expression "ductile state" means that the heat
content of the film web is such that the film can be stretched. The film
~0 is in this state, e.g., when it is stitl warm as a consequence of a heat-
ing step precedlng extrusion or another type of pre-treatment, and thus
may be easily deformed. In this case, the bearing pressure required for
appllcation of the embossed lines may be maintained particularly low.
In a further embodiment of the inventive process, additional
embossed lines extending parallel to each other are applied in the longi-
~dlnal directlon. Such a measure is of advantage when very wide webs
are to be wound up, because the additional embossed lines effect a
ihrther spaclng of the layers of film from each other.
If a wide film web is to be later subdivided into narrower webs,
lt is preferred to apply a pair of very close embossed lines in a manner
uch that the center line between the two embossed 11nes coincides with
the area where the division is to take place. The distance between the
two embossed lines forming the pair is preferably in the range of from 2
to 50 mm.
Although the process according to the invention may be applied
;' ~ to fllms generally, especially to thermoplastic films such as films of
olefin polymers, e.g. polyethylene, polypropylene and the like, or
polyesters, e.g. polyethylene terephthalate, or their mixed and
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copolyesters and further to films of polyvinyl chloride, polystyrene,
or polyamides and the like, it was found to be particularly suitable
for films which were improved by stretching. In particular the
polypropylene, polyamide, polyvinyl chloride or polyethylene tere-
phthalate films which are presently widely used assume optimum
physical properties by mono- or biaxial stretching. In the case of
these films, the process of the invention is preferably conducted in
such a way that the embossed lines are applied prior to the stretching ~ -
operation, which may be either in the longitudinal or in the trans-
verse direction. Alternatively, it is possible to apply the embossed
lines immediately following the stretching process itself, because,
during stretching, the film is heated to the stretching temperature
and thus is in the ductile state. P~eferably, the embossed lines
are applied immediately following a transverse stretching process,
because during the stretching process the film web is subjected to
tensional forces acting on the edges of the web in the direction of
the plane of the web, e.g. by means of clip chains.
According to another embodiment, also within the scope of
the present invention, the film web is first subjected to one or more
stretching operations and the embossed lines are applied some^time
later. In this case, it is preferred to apply the lines after a
longitudinal/transverse stretching process. Then, a further stretch-
ing process may take place, e.g. an after-stretching in the longitu-
dinal direction. In the case of stretched films, e.g. polypropylene,
polyester or polyamide films, the thermal treatment performed while -
the web is tensioned in all directions - which is also an essential
feature of the present invention - is preferably performed at a
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temperature above the temperature of the extruded film, because in
~ this manner the desired final stage with respect to residual shrink-
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: 30 age, for example, is reached within a very short time.
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In the case of unstretched webs of film and ln the case of cer-
tain other films, e.g. monoaxially stretched films, the thermal treat-
ment is performed at temperatures below the temperature of the extruded
film .
The thermal treatment may be performed at a constant tempera-
ture. In some cases, however, it is advisable to perform the thermal
treatment while the temperature rises or drops, in order to impart certain
desirable physical characteristics to the finished film.
Normally, the thermal treatment ls performed while the tension
,~ 10 remalns constant. Especially in the case of relatively deep embossed
llnes, however, it was found to be preferable to increase the tenslon,
preferably ln the transverse direction.
A~though the width of the embossed lines is not critical, it is
of advantage for the embossed lines to have a width of 1 to 30 mm,
preferably 5 to 15 mm, in order to achieve as high as possible a yleld
of film for further processing.
; I.ikewise, there is no absolutely crltlcal value for the depth
o~ the embossed llnes . Preferably, however, the lines have a depth
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~` corresponding to 1 to 20 per cent, preferably 5 to 10 per cent of the
thlckness of the film.
It may be desirable, in some cases, for the embossed lines
I ~ to be applied with interruptions, for example when the web is to be
severed after a certain distance. The process of the invention is also
suitable for this purpose, because the embossed lines may be applied
in such a manner that certain areas remain unembossed and the lines
are interrupted.
The present invention also covers a wound roll produced by
windlng webs of film treated in accordance with the process of the
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invention. In partlcular, the present inventlon covers wound rolls pro-
duced by winding webs of film ranging in thickness from 2 to 100 ~m,
preferably from 10 to 50~m.
It will be obvious to those skilled in the art that many modifi-
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cations may be made within the scope of the present lnvention without
departing from the spirit thereof, and the invention includes al~ such
modiflcations .
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