Note: Descriptions are shown in the official language in which they were submitted.
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HOECHST TRESPAPHAN GMBH HOE 95/P 002 DCh.VK/As
Description
5 Process for longitudinal stretching in the production of oriented polypropylene films
The invention relates to a process for longitudinal stretching of biaxially oriented
films made from thermoplastic polymer, in particular polypropylene. The films
10 produced by the process are distinguished by good heat-sealability, good
appearance and a good thickness profile.
The prior art discloses various processes for the production of biaxially oriented
polypropylene films (BOPP films). In the stenter process, the BOPP film is produced
15 by extruding the melt through a flat-film die followed by stretching in the longitudinal
and transverse direction. The film generally has a multilayer structure.
In detail, the process involves first compressing, warming and melting the polymers
in an extruder. The melts corresponding to the individual layers of the film are jointly
20 filtered and forced simultaneously through a flat-film die, giving a melt film as
extrudate. The melt film is cast onto a chill roll, where it solidifies to give an
unoriented film. The film is subsequently oriented in the longitudinal direction via
rolls and in the transverse direction in a stretching oven and is subsequently heat-
set. The film may subsequently be surface-treated by flame or corona. The film is
25 wound up and finished to give the customer-ready cut roll.
Biaxially oriented polypropylene films are distinguished by a very good propertyprofile. Their characteristic properties are high mechanical strength, good heat-
sealability and a bright appearance. Owing to this good property profile and
30 excellent processing properties - characterized by low slip, high rigidity and good
thickness profile - BOPP films have been used in a wide variety of applications. The
most important market segment is packaging, which accounts for about 80% of the
films produced. In addition, BOPP films are employed in significant amounts in
technical applications, for example in metallization and transfer metallization,
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lamination and as electrical insulation in capacitor foils. Electrical foils generally
have a thickness of less than 10 ,um. The essential prerequisite for flaw-free
processing of these very thin foils is a very good thickness profile.
5 The trend in the production of BOPP films is toward higher production speeds and
wider production widths. In 1980, the machine speed at which BOPP films were
produced was 100-150 m/min, while today it is 300-400 mtmin. The machine width in
1980 was about 5 m, while today it extends up to 8 m. An increase in the production
speed and machine width also means an increase in the stretching rate, i.e. the rate
10 at which the film is oriented.
An increase in the stretching rate means greater mechanical and thermal stresseson the thermoplastic during orientation. In particular during longitudinal stretching,
where the stretching of the film is carried out in a very short distance between two
15 rolls rotating at different speeds, the increase in the stretching rate can result in an
impairment in the film quality.
During longitudinal stretching, the film web is first passed over a plurality of heated
rolls which bring the material to the temperature necessary for stretching. These
20 rolls are driven at a low peripheral speed. The film then reaches one or more chill
rolls, which are driven at a higher peripheral speed than the heating rolls. Thedifferent speeds of the heated and chill rolls produces longitudinal stretching of the
film.
DE-B 1 221 786 describes an apparatus for stretching a thermoplastic film web inthe longitudinal direction. In this apparatus, two rolls which press the edges of the
film web against a support are provided after, regarded in the stretching direction,
the region warmed by the heating device. These undriven rolls have the task of
countering any reduction in the film web width during the longitudinal stretching. The
stretching force in this process is applied by the rolls to one side of the film.
DE-B 1 212 290 describes an apparatus for longitudinal stretching of a material web
in which the material web is heated to the stretching temperature by heating rolls in
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the longitudinal direction and subjected in the stretching region to a tensile force
acting in the longitudinal direction. The transport devices for the material web are
ar!anged in such a way that they keep the tensile force away from the part of the
material web Iying on the heating roll. In an expedient embodiment of the invention,
5 the transport devices are formed by a transport roll driven independently of the
heating roll and by an undriven counterpressure roll. The process described has the
disadvantage that the tensile force is again only applied to one side of the film.
A particular embodiment of the device of DE-B 1 212 290 is described in German
Patent 1 504 058. The device of DE-B 1 212 290 is refined in such a way that theoptimum length of the stretching zone can be set precisely depending on the typeand thickness of the material web, the operating speed and the desired stretching
ratio. This is achieved in accordance with the invention in that, in order to change
the length of the stretching zone, the pair of transport and/or the pair of tension rolls
15 is/are pivotable about the axis of the heating roll or of the chill roll. This process
again has the disadvantage of application of force on one side and an unfavorable
arrangement of the chill rolls 8 and 9.
German Patent 1 919 299 claims a process for the longitudinal stretching of a
20 plastic film in which longitudinal tension is kept away from the final heating roll in a
particularly simple manner. The object is achieved in accordance with the invention
in that a temperature drop of from 2 to 25C, depending on the film material, isproduced between the temperature of the final heating roll and the temperature of
the first stretching roll. This process again has the disadvantage that the stretching
25 force is in each case applied to only one film surface, albeit in an alternating
manner. In addition, the unfavorable arrangement of the rolls 7 and 8 means that the
risk of air bubbles between the film and the rolls 7 and 8 is particularly great.
DE-B 2 833 189 describes a process for the longitudinal stretching of an at least
30 two-layer plastic film, in which the layer of higher melting point is oriented and the
layer of lower melting point is heat-sealable. Before reaching the stretching rolls, the
film is first warmed to a temperature which is both above the stretching temperature
of the higher-melting layer and below the adhesive temperature of the lower-melting
2192917
layer. The lower-melting layer is then shock-cooled within a short time interval to a
temperature significantly below the adhesive temperature, while the higher-melting
layer remains at the stretching temperature. This procedure prevents adhesion ofthe film sur~aces to the stretching rolls. This process again has the disadvantage
that the force for longitudinal stretching of the film is only applied to one side of the
films.
The known BOPP production processes have the disadvantage of r.a Ionger giving
high performance if the production speeds are increased. At increased productionspeeds, these processes give films having an uneven thickness profile, impaired
heat-sealing properties and a worse film appearance.
The object of the present invention was therefore to provide a process for the
longitudinal stretching of a thermoplastic film by means of which a film having good
heat-sealability, good appearance and a good thickness profile is provided at the
production speeds customary today.
The object is achieved in accordance with the invention by a process for the
longitudinal stretching of an at least single-layer thermoplastic film. In the novel
process (the reference numbers below refer to Fig. 1), before the longitudinal
stretching, the film is warmed in the slow-running part of the stretching machine to a
temperature suitable for stretching and fed to a stretching zone (10). In the process,
a) the slow-running part of the stretching machine contains the driven roll (1),
and the fast-running part of the stretching machine contains the driven roll
pair (2)/(3), the roll pair (2)/(3) being arranged in such a way that a roll nip (4)
is formed, and
b) the film (9) is passed into the roll nip (4) in such a way that it is grippedsimultaneously or virtually simultaneously by the roll pair (2)/(3) at the contact
points (5)/(6), and the film (9) is stretched between the roll (1 ) and the rollpair (2)/(3).
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In accordance with the invention, the roll pair (2)/(3) is arranged in such a way that
the film is gripped virtually simultaneously at the contact points (5)/(6), causing the
stretching force in the fast-running part of the stretching machine to be applied to
both film surfaces 7 and 8. This produces better tension distribution within the film 9,
5 in particular within the layers of the film 9 close to the surface, compared with
application of the stretching force on one side. Surprisingly, the fact that the rolls (2)
and (3) are driven and their novel arrangement, i.e. application of the str~Lching
force to both sides, means that the properties of the film are significantly better. This
is particularly true of the optical properties and the heat-sealing properties of the
1 0 film.
It has been found that, in the longitudinal stretching processes of the prior art (cf.
Figure 2), the entire stretching force is taken up by only one film surface (Figure 2,
film surface 7), i.e. the force is applied to one side, which can result in a critical
15 stress value in/on the film being reached or exceeded in the layer of the film close to
the surface. The stress peaks of this type can result in damage to the surface.
The fact that the film thickness in the fast-running part of the stretching machine is
less by the stretching ratio f (f = v2/v1 ) means that a critical stress value initially
20 occurs in the fast-running part of the longitudinal stretching machine. The stresses
within the film in the slow-running part of the stretching machine are significantly
smaller owing to the greater thickness, so that the risk of surface damage to the film
is significantly less there. In accordance with the invention, application of force to
both sides by the rolls 2 and 3 in the fast-running part of the stretching machine is
25 therefore sufficient to improve the appearance, thickness profile and heat-sealing
properties.
In addition, the simultaneous contact of the film surfaces 7 and 8 with the rolls 2 and
3 at the points 5 and 6 ensures that the air dragged along by the film web 9 is
30 squeezed out at the contact points 5 and 6. The film is in contact with the rolls 2 and
3 over its entire surface, so that optimum force application is ensured between the
rolls 2 and 3 and the film surfaces 7 and 8. If, by contrast, the roll 3 is positioned as
shown in Figure 3, air is dragged in between the film 9 and the roll 2, forming an air
2 ! 9291 7
cushion which reduces the contact area for force application. The film 9 is no longer
flat on the roll 2, causing the partial interruption in force application between the film
9 and the roll 2. This results in film sections being accelerated to different extents in
the stretching direction in this region of the roll, which causes width and thickness
differences in the film 9. In production of the film by the novel process as shown in
Fig. 1, the inclusion of air is prevented by the film being gripped virtually
simultaneously at the contact points 5 and 6 by the rolls 2 and 3. It has been found
that the thickness profile of the film is consequently significantly better than in an
arrangement as shown in Figure 2 or 3.
The invention is explained in greater detail below with reference to drawings.
Figure 1 is a diagrammatic view of the embodiment according to the invention.
Figure 1 A shows a detailed view on the area I A of Figure 1.
Figures 2 and 3 show illustrative embodiments of longitudinal stretching devicesaccording to the prior art and non-inventive embodiments.
Figure 4 A and B shows an embodiment according to the process wherein roll 3 is
driven with the aid of roll 2.
Figures 5 and 6 show further embodiments of the process according to the
invention.
Figure 7 shows an instrument used to conduct a hot tack test.
The 'ongitudinal stretching machine shown in Figure 1 comprises, in accordance
with the invention, the three driven rolls 1, 2 and 3 by means of which the film 9 is
stretched by the longitudinal stretching ratio f. The speeds of the rolls are v1, v2 and
V3. The longitudinal stretching ratio f is given, for frictional contact between the rolls
1, 2 and 3 and the film 9, approximately by the ratio of the speeds v2 and v1 f the
rolls 2 and 1. The longitudinal stretching ratio f for PP films is usually in the range
from 4 to 9, preferably in the range from 4.5 to 8Ø When the stretching zone 10 is
reached, the film 9 has achieved a temperature Ts at which it can be stretched by
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the particular stretching ratio. The length of the stretching zone is generally from 50
to 600 mm, preferably from 50 to 500 mm, in particular from 50 to 400 mm.
Depending on the raw material, film thickness, stretching rate and stretching ratio,
the temperature Ts is preferably between 80 and 160C. The stretching force Es is
5 applied to the film 9 by means of friction between the driven roll 1 and the film
surface 7 and between the driven roll pair 2/3 and the film surfaces 7 and 8. The
speeds v2 and V3 of the roll pair 2/3 are generally the same, but can also differ
slightly from one another by a maximum of 5%. The rolls 2 and 3 can be driven
separately, although joint driving of the rolls 2 and 3 is also possible. Figure 4
10 shows diagrammatically an example of a joint drive of the roll pair 2/3. To this end,
the roll 3 is expediently designed in such a way that the two rolls 2 and 3 are in
contact in the edge region 13. The film 9 lies flat on the two roll surfaces in the
recess 14 and is transported by friction forces. If only one film thickness is produced
on the machine, the roll pair 2/3 can be made of a hard material, for example steel
15 with a chrome-plated or ceramic surface. Somewhat greater flexibility with respect to
the film thickness is achieved if, for example, the roll 3 is rubber-covered.
Application force is particularly good if the rubber has a Shore A hardness of
between about 50 and 100. In the production of greatly different film thicknesses,
the roll 3 must be replaced, which is expediently carried out when re-setting the
20 thickness. The two-sided frictional contact between the roll pairs 2 and 3 and the
film surfaces 7 and 8 means that the film surface 7 is subjected to significantly lower
stresses than in the case of frictional contact on one side, i.e. in arrangements with
only one driven roll in the fast-running part of the stretching machine. In the
invention, the two-sided frictional contact is achieved in the fast-running part of the
25 stretching machine since the stresses acting on the film are greatef there than in the
slow-running part of the stretching machine owing to the lower film thickness. With
regard to achieving a critical stress value, the slow-running part of the stretching
machine is insignificant.
30 In spite of force application on both sides by the novel process, the stretching force
is not necessarily applied equally by the rolls 2 and 3. The further looping of film 9
around roll 2 means that the distribution of the stretching force onto rolls 2 and 3 is
not equal. Experiments have shown that the stretching force in the stretching
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arrangement shown in Fig. 1 (film - roll loop angle 50 to 180) can be divided
between rolls 3 and 2 in a ratio of up to 40:60. In spite of the unequal distribution of
the force application, the stress in the film is lower in the novel process than in one-
sided stretching force application.
A further feature of the invention is that the roll pair 2/3 is arranged in such a way
that the air dragged along by the film 9 is squeezed out at the contact points 5 and 6
of the roll pair 2/3. To this end, the film surfaces 7 and 8 are gripped simultaneously
by the roll pair 2/3 at the contact points 5 and 6 and transported further by friction.
10 This ensures that the film 9 lies flat on the rolls 2 and 3 without air being dragged in
and air cushions forming between the film 9 and the roll surfaces. The contact points
should not be more than 50mm, preferably not more than 40 mm, in particular not
more than 20mm, measured in the film web direction, apart from one another. The
risk of air inclusion increases with machine speed, since the amount of air dragged
15 by the film 9 increases approximately proportionally with the film web speed.
It is observed that positioning of the rolls 2 and 3 as shown in Figure 3 causes the
film 9 to run roughly as far as the contact point 5 on the roll 2. The film web width is
unstable, i.e. it is subject to variations. The air included between the film 9 and the
20 roll surface escapes suddenly after a critical pressure has built up in the air cushion.
The rough, unstable film running results from local different accelerations of the film
web. Thick/thin areas are initiated in the film 9, resulting in a worse thickness profile
of the film 9.
25 Figure 5 shows a further expedient embodiment of the invention. The stretching
machine comprises in accordance with the invention the three driven rolls 1, 2 and 3
and the novel arrangement of the roll pair 2/3. The slow-running part of the
stretching machine additionally comprises a nip roll 15, which serves further toimprove the frictional contact in the slow-running part of the stretching machine. The
30 nip roll can be driven or undriven. For the outlined reasons, the nip roll isexpediently arranged at the contact point of the film 9 with the roll 1, so that,
analogously to the roll pair 2 and 3, the film comes into contact simultaneously or
virtually simultaneously with roll 1 and the nip roll. The looping of the film 9 around
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the rolls 1 and 2 and the diameter of the roll 3 are advantageously selected so that
the shortest possible stretching zone 10 can be achieved (S-shaped loop).
This is important, for example, for the production of films which are sensitive to
splitting or which can only be stretched at a low longitudinal stretching ratio.
Figure 6 shows a further embodiment of the invention in which longitudinal
stretching is divided over a plurality of stretching zones 10 to 12. In the embodiment
shown, the rolls 16 to 19 are likewise driven. By means of the selected arrangement
of the rolls, the individual stretching ratios can be matched in a targeted manner to
10 the raw material, and the force application and the associated stresses on the two
film surfaces can be divided equally.
The longitudinal stretching in accordance with the invention is particularly
advantageously used in the production of films by the stenter process, which is
15 known per se. It is advantageously suitable both for the production of films which
are only monoaxially oriented (only in the longitudinal direction), and for the
production of biaxially oriented films.
In the stenter process, the melts corresponding to the individual layers of the film
20 are extruded or coextruded through a flat-film die, the resultant melt film is taken off
on one or more roll(s) for solidification, the film is subsequently stretched and heat-
set and, if desired, flame- or corona-treated.
If desired, the film can be biaxially oriented in the longitudinal and transverse
25 directions. The biaxial stretching (orientation) is carried out successively, it being
preferred to carry out the stretching by the novel process first in the longitudinal
direction of the machine and then in the transverse direction of the machine.
Firstly, as usual in the extrusion process, the polymers or the polymer mixtures of
30 the individual layers are compressed and liquefied in extruders, it being possible for
the additives already to be present in the polymers or in the polymer mixtures. The
melts are then forced jointly and simultaneously through a flat-film die, and the
extruded, single- or multilayer melt film is taken off on one or more take-off rolls,
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where it is cooled and solidified. The resultant film is then stretched longitudinally
and preferably also transversely to the extrusion direction, which results in
orientation of the molecule chains. The longitudinal stretching is carried out by the
novel process described, and the transverse stretching is carried out with the aid of
5 an appropriate tenter frame. The longitudinal stretching ratios are in the range from
4 to 9, preferably from 4.5 to 8.0, and the transverse stretching ratios are in the
range from 7 to 12, preferably from 8 to 11.
The stretching of the film is followed by heat-setting (heat treatment), during which
the film is kept at a temperature of from 80 to 160C for from about 0.1 to
10 seconds. This can be followed by printing pretreatment, for example by means of
a flame or electrical corona process. The treatment intensity is generally in the
range from 36 to 50 mN/m, preferably from 38 to 45 mN/m. The film produced in this
way is wound up in a conventional manner using a wind-up device.
It has proven particularly favorable to keep the take-off roll or rolls by means of
which the extruded melt film is cooled and solidified at a temperature of from 10 to
120C, preferably from 20 to 100C, by means of a heating and cooling circuit. The
temperatures at which the longitudinal and transverse stretching are carried out can
20 vary within a relatively broad range and depend on the particular composition of the
individual layers and on the desired properties of the film. In general, the
longitudinal stretching is preferably carried out at from 80 to 160C and the
transverse stretching is preferably carried out at from 120 to 1 70C.
25 The film produced by the novel process can have a single-layer or multilayer
structure. It can be non-heat-sealable or heat-sealable. In a particular embodiment,
the film has outer layers on both sides of its base layer. In a further embodiment, the
film has at least one interlayer, if desired on both sides of its base layer.
30 The base layer of the film generally contains at least 85% by weight, preferably from
90 to 99% by weight, in each case based on the base layer, of a thermoplastic
polymer described below and, if desired, additives in effective amounts in each
case.
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Suitable thermoplastic polymers are polymers of olefins having 2 to 10 carbon
atoms, preferably polypropylenes, polyethylenes and/or polybutylenes.
Thcrmoplastic polymers can also be polyethylene terephthalates, polybutylene
terephthalates and other polyester derivatives. The novel process is preferably
5 suitable for the production of films having a polypropylene base layer. Of propylene-
containing polymers, particular preference is given to propylene homopolymers.
Suitable propylene polymers have a melting point of from 120 to 1 65C, preferably
from 140 to 162C, and a melt flow index (measurement in accordance with DIN 53
735 at 21.6 N and at 230C) of from 1.0 to 10 9/10 min, preferably from 1.5 to 6 9/10
min. Propylene polymers contain at least 80% by weight, preferably from 90 to 100%
by weight, in particular from 95 to 100% by weight, of propylene units. The n-
heptane-soluble content of the propylene polymer is generally from 1 to 10% by
weight, based on the starting polymer. Particular preference is given to propylene
homopolymers whose n-heptane-insoluble content is isotactic. The chain isotacticity
15 index, determined by 13C-NMR spectroscopy, is greater than 85%, preferably
greater than 90%. The molecular weight distribution of the propylene homopolymercan vary within broad limits, depending on its area of application. The ratio between
the weight average molecular weight Mw and the number average molecular weight
Mn is generally between 2 and 15.
If the film is opaque, the base layer additionally contains vacuole-initiating fillers, for
example CaC03 or incompatible polymers and/or pigments, as described in the prior
art. In addition, the base layer may additionally contain conventional additives, such
as antistatics, antiblocking agents, lubricants, stabilizers, neutralizers, pigments
25 and/or nucleating agents in effective amounts in each case.
If an electroinsulation film (capacitor foil) is produced by the novel process, the raw
material used must have high purity. A high-purity polypropylene of this type must
have a low ash and chlorine content compared with the packaging raw material and30 must have the lowest possible content of ionogenic constituents. Electroinsulation
film is therefore usually not provided with antistatics and lubricants. The chlorine
content of the high-purity polypropylene is less than 50 ppm, and the residual ash
content less than 70 ppm.
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12
A preferred embodiment of the oriented polypropylene film produced by the novel
process comprises at least one outer layer, preferably on both sides, of olefinshaving 2 to 10 carbon atoms. Interlayers of olefinic polymers may additionally be
applied.
Examples of olefinic polymers of this type for the outer layer and/or interlayer are
a propylene homopolymer
a copolym~?r of
ethylene and propylene or ethylene and 1-butylene or
propylene and 1-butylene, or
a terpolymer of
ethylene and propylene and 1-butylene, or
a mixture or blend of two or more of said homopolymers, copolymers and
terpolymers, particular preference being given to propylene homopolymers or
random ethylene-propylene copolymers having an ethylene content of from 1 to
10% by weight, preferably from 2.5 to 8% by weight, or random propylene-1-
butylene copolymers having a butylene content of from 2 to 25% by weight,
preferably from 4 to 20% by weight, in each case based on the total weight of the
copolymer, or random ethylene-propylene- 1-butylene terpolymers having an
ethylene content of from 1 to 10% by weight, preferably from 2 to 6% by weight, and
a 1-butylene content of from 2 to 30% by weight, preferably from 4 to 20% by
weight, in each case based on the total weight of the terpolymer, or a blend of an
ethylene-propylene-1-butylene terpolymer and a propylene-1-butylene copolymer
having an ethylene content of from 0.1 to 7% by weight and a propylene content of
from 50 to 90% by weight and a 1-butylene content of from 10 to 40% by weight, in
each case based on the total weight of the polymer blend.
The propylene homopolymer employed in the outer layer and/or interlayer containsfrom at least 98 to 100% by weight of propylene and has a melting point of 140C or
above, preferably 150 to 170C, preference being given to isotactic
homopolypropylene. The homopolymer generally has a melt flow index of from 1.5
to 20 9/10 min, preferably from 2.0 to 15 g/10 min.
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The above-described copolymers and terpolymers employed in the outer layer
and/or interlayer generally have a melt flow index of from 1.5 to 30 g/10 min,
preferably from 3 to 15 g/10 min. The ,nelting point is in the range from 120 to140C. The above-described blend of copolymers and terpolymers has a melt flow
index of from 5 to 9 g/10 min and a melting point of from 120 to 150C. All the melt
flow indices given above are measured at 230C and a force of 21.6 N (DIN 53 735).
If desired, the outer layers and/or interlayers can likewise contain antistatics,
antiblocking agents, lubricants, neutralizers, stabilizers and hydrocarbon resins.
The overall thickness of the polypropylene film produced by the novel process can
vary within broad limits and depends on the intended application. It is preferably
from 2 to 120 ,um, in particular from 2.5 to 100 ,um, especially from 2.5 to 80 ,um, the
base layer making up from about 40 to 100% of the overall film thickness.
The thickness of the outer layers is greater than 0.1 ,um and is preferably in the
range from 0.2 to 5 ,um, in particular from 0.3 to 1.5 ,um, it being possible for the
outer layers to have identical or different thicknesses. The thickness of any
interlayer(s) present is greater than 0.1 ,um and is preferably in the range from 0.5 to
15 ,um, in particular from 0.7 to 10 ,um.
The film produced by the novel process is particularly suitable for processing on
high-speed machines, for example for packaging, lamination or metallization. It has
all the important properties required of oriented polypropylene films with respect to
25 their different applications. In particular, it has good heat-sealability, a good
appearance anc an excellent thickness profile.
The following measurement methods were used to characterize the raw materials
and the films:
Melt flow index
The melt flow index was measured in accordance with DIN 53 735 at a load of 1.6 N
and at 230C.
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14
Melting point
DSC measurement, maximum of the melting curve1 heating rate 20C/min.
Molecular weight determination
5 The mean molecular weight is determined by three-detector gel permeation
chromatography. The substance is dissolved in an eluent, such as THF1 and applied
to a separating colun ,n. The separating column has a length of 90 cm and is filled
with a porous support material whose pore size is 5 ,um. Detection is by UV
absorption spectroscopy at various wavelengths and by means of the refractive
10 index and light scattering capacity of the fractions. The calibration is carried out by
means of a standard compound of known molecular weight. Comparison of the UV
absorption of the standard substance with the absorption of the sample enables
assignment of the molecular weights.
15 Determination of the minimum sealing temperature (MST)
The minimum sealing temperature is determined by the peel method. Heat-sealed
samples (seal seam 20 mm x 150 mm) are produced using a Brugger HSG/ET
sealing machine by sealing a film at various temperatures with the aid of two heated
sealing jaws at a sealing pressure of 15 N/cm2 for 0.5 second. Test strips with a
20 width of 15 mm are cut out of the sealed samples. The T-seal seam strength1 i.e. the
force necessary to separate the test strips1 is determined using a tensile tester at a
peel rate of 200 mm/min, the seal seam plane forming a right angle with the
direction of tension. The minimum sealing temperature is the temperature at which a
seal seam strength of 0.5 N/15 mm is achieved.
25 In addition1 the tear strength is measured at a sealing temperature of 130C -
likewise by the peel method.
Hot tack
Hot tack denotes the strength of a still-hot seal seam immediately after opening of
30 the sealing jaws. In the test used1 the separation in mm experienced by the heat-
seal seam at a load of 1 N (seal seam width 30 mm) is measured. The test is carried
out at a sealing temperature of 1 50C. The measurement is a Hoechst internal
standard. There are no corresponding DIN and ASTM standards. The test
21 92~17
instrument used is the heat contact sealing unit with grooved sealing jaws (20, 21)
and deflection rolls (R1, R2, R3) for a separation angle of 180 (cf. Figure 7). The
sealing time is 0.5 second and the sealing pressure is 30 N/cm 2
In the measurement process, the measurement strips (22) having a width of 30 mm
are laid one on top of the other and fixed at the ends with a weight of 1 N. A flat
spatula (23) is inserted between the film layers, and the measurement strips are fed
around the deflection rolls (R1, R2, R3) over the sealing jaws and clamped such that
the flat spatula will be positioned between the grooved sealing jaws. After
completion of the sealing, the jaws open automatically. The sealed measurement
strip is jerked forward to the deflection roll by the weight of 1 N (G), where the seal
resulting from the sealing jaws (21) is separated at a separation angle of 180. The
depth of the lamination of the seal seam in mm is assessed. The greater the
numerical value, the worse the hot tack. The sealing of sealing jaws (20) remains
unaffected during determination of the hot tack.
Gloss
The gloss was determined in accordance with DIN 67 530. The reflectometer value
was measured as a characteristic optical parameter for the surface of the film. In
accordance with the ASTM-D 523-78 and ISO 2813 standards, the angle of
incidence was set at 20 or 60. A light beam hits the planar test surface at the set
angle of incidence and is reflected or scattered thereby. The light beams hitting the
photoelectronic receiver are indicated as proportional electrical quantities. The
measurement value is dimensionless and must be indicated together with the angleof incidence.
Hase
The haze of the film was measured in accordance with ASTM-D 1003-52.
Assessment of the thickness profile
The thickness profile was measured on-line by traversing a measurement head overthe film web width (F+H or FAG measuring instrument). The measurement head
contains a beta-emitter which measures the absorption in the film and converts the
2 1 929 1 7
16
value into a corresponding thickness value. The thickness values are measured
over the film width and plotted as a graph. The thickness profile is assessed using
the so-called R value. This is calculated from the ratio between the difference of
maximum thickness value and minimum thickness value and the mean thickness of
5 the film.
maximum thickness - minimum thickness
R value =
mean thickness
The thickness profile is better the lower the R value.
Although the present invention has been described with preferred embodiments it is
to be understood that modification may be resorted without departing from the spirit
15 and scope of this invention as those skilled in the art would readily understood.
The invention is described in greater detail now with reference to examples.
Example 1
20 A transparent, heat-sealable, three-layer film with a symmetrical structure and an
overall thickness of 40 ,um was produced by coextrusion followed by stepwise
orientation in the longitudinal and transverse directions. The outer layers each had
a thickness of 0.6 ,um.
25 Base layer A:
99.70% by wt. of isotactic polypropylene from Solvay with the tradename Eltex
PHP 405
0.150% bywt. of N,N-bisethoxyalkylamine
0.150% by wt. of erucamide
Outer layers B:
99.67% by wt. of random ethylene-propylene copolymer having a C2
content of 4.5% by weight
0.33% by wt. of SiO2 as antiblocking agent having a mean particle size of 4 ,um.
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The production conditions in the individual process steps were as follows:
Extrusion: Temperatures: Layer A 280C
Layers B 280C
Temperature of the take-off roll 30C
Longidutinal stretching: Temperature 130C
Longitudinal stretching ratio 5
Transverse stretching: Temperature 160C
Transverse stretching ratio 10
Heat-setting: Temperature 1 40C
Convergence 15 %
Film web speed 230 m/min
The film was produced by the novel process as shown in Figure 1. The properties of
the films from the examples and comparative examples are shown in the table
20 below.
Example 2
A film was produced by the novel process analogously to Example 1.
25 Compared with Example 1, the random copolymer in the outer layers was replaced
by a propylene homopolymer (Eltex PHP 405). The film is not heat-sealable.
The process parameters were the same. The film is distinguished by very good
optical properties and a very good thickness profile.
30 Example 3
A film is produced by the novel process analogously to Example 1.
Compared with Example 1, a single-layer film was produced for electrical insulation.
The polymer ,'or this film was a high-purity isotactic propylene homopolymer from
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18
Borealis with the tradename Borealis VB 2142 E, Melt flow index = 2.2 9/ 10 min.The thickness of the film was 3.5 ~m. The process conditions during production of
the film which had changed compared with Example 1 were as follows:
Extrusion: Temperature: 270C
Temperature of the take-off roil 90C
Long;tudinal stretching: Temperature: 150C
Longitudinal stretching ratio: 5.5
The high take-off roll temperature of 90C and the special longitudinal stretching
temperature gave a film having a rough surface, as described, for example, in EP-A
0 497 160. The film is distinguished by an excellent thickness profile.
15 Comparative Example 1
Compared with Example 1, the longitudinal stretching of the film was now carried out
as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The
hot tack and the optical properties of the film are significantly worse, and thethickness profile of the film has become worse.
Comparative Example 2
Compared with Example 1, the longitudinal stretching of the film was now carried out
as shown in Figure 3, where the contact points are more than 100 mm apart and the
film is no longer gripped simultaneously by the driven rolls 2 and 3. The hot tack and
25 optical properties of the film are worse, and the thickness profile of the film has
become significantly worse.
Comparative Example 3
Compared with Example 2, the longitudinal stretching of the film was now carried out
30 as shown in Figure 1, but without driving of the roll 3, i.e. only roll 2 was driven. The
optical properties of the film are significantly worse, and the thickness profile has
become worse.
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19
Comparative Example 4
Compared with Example 2, the longitudinal stretching of the film was now carried out
as shown in Figure 3, where the contact points are more than 100 mm apart. The
optical properties of the film are worse, and the thickness profile has become
5 significantly worse.
Comparative Example 5
Compared with Example 3, the longitudinal stretching of the film was now carried out
as shown in Figure 1, but without driving of roll 3, i.e. only roll 2 was driven. The
10 thickness profile of the film has become worse.
Comparative Example 6
Compared with Example 3, the longitudinal stretching of the film was now carried out
as shown in Figure 3, where the contact points are more than 100 mm apart. The
15 thickness profile of the film has become significantly worse.
