Note: Descriptions are shown in the official language in which they were submitted.
1038Z46
The present invention is concerned with a process and an
equipment for coating a base, especially a strip of paper or paper
lined with metal foil, with a thcrmoplastic, in which the
thermoplastic material is extruded to a thin film which, while it
i9 still warm, is brought into contact with the base and, with
simultaneous cooling to a temperature below the softening
.
termperature of the plastic material, is pressed to the base.
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In the production, of for example, coating material for
- packing purposes, it often happens that a layer of thermoplastic
- 10 material, for example polyethylene, is applied to a base where
the thermoplastic material is extruded to a thin film which, while
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it is stiil in the molten or semi-molten state, is united with the
, base which may, for example, consist of a strip of cardboard or a
metal foil. - ~
~ In order to achieve good adhesion between the extruded
- - - 15 thermoplastic coating and tne base, the base and thermoplastic
coatir~ are pressed together between cooled pressure rollers,
-~ whereby the thermoplastic layer is brought into good contact with
- the base and at the same time stabilised by cooling~ When carrying
out this coating process it often happens that the plastic material
- 20 adheres to the cooling roll and that the coating material is ir. this
- ~ - case subjected to a force that is mainly at right angles to the
coating surface. ~his force can easily cause delamination of the
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~ aterial that need not be confined to the boundary zone of the
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applied plastic layer but the delamination can be localised in the
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- 25 base, for example if this consists of paper or other cellular
- material. i-
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103t~Z46
~ ttelnpts have been made to avold the above mentloned
difficulty by wetting the cooling roll with water as well as by
applying a coating of polytetrafluoroethylene, sold under the trade
mark "Teflon", to the outer surface of the roll. The best result
is achieved with the "Teflon" coated roll but delamination can
occur even with such a cooling roll; moreover it has been found
that the Teflon material cannot withstand for sufficiently long
the mechanical and thermal stresses which arise at the outer sur-
face of the cooling roller.
According to the present Invention, there is provided a
process for the coating of a substrate, consisting of paper or
paper lined with a metal foil, with polyethylene in which the poly-
- ethylene is extruded to a thin film which, while it is still warm,
is brought into contact with the substrate and, with simultaneous
cooling to a temperature below the softening temperature of the
polyethylene, is pressed together with the substrate by passing the
substrate and the extruded polyethylene film through cooperating
rolls wherein at least one roll which is brought into contact with
the polyethylene coating is moistened on its rolling surface with
a liquid having a critical surface tension below 73 dyne/cm and a
boiling point below 100C, and consisting of a mixture of water and
between about 10% and about 50% ethyl alcohol, whereby an uninter-
~ rupted liquid layer is applied to the whole contact surface of the
-- roll or rolls with the polyethylene coating.
The invention is described below by way of example with
reference to the enclosed schematic drawing and the photographic
representation.
Fig. 1 is a schematic sketch of a coating apparatus.
- 3 -
1038Z46
- ~ie~ 2 is a di~gram showine how the adhesion between cooling
roller and thermcplastic coating varies according to the critical
surface tension of the coolin~ roller. I
Fig. 3 i8 a photographic enlargement of a cross-section
through a coating material which has delaminated.
- Fig 4 is a photo~aphic enlargement of a cross-section through
a coating produced in accordance with the invention.
Fig. 5 is a photographic enlargement of the surface layer of
the outer plastic coat of a coating material the coating process of
which has been carried out in the usual way using a steel cooling
- rol 1 .
t
~ig. 6 is a photographic enlargement of the surface layer of an
- outer plastic layer of a coating material produced in accordance with
~', - the invention.
In order to clarify the matter, the concept of critical surface
tensions will be discussed in more detail.
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It can be said that the critical surface tension of a material
. .
- is a measure of the tendency to wetting, where the wetting tendency
is grea~er at a lower critical surface tension than at a higher. The
physical measure of critical surface tension is expressed in dyne/cm
and the size of the critical surface tension can be determined
.. .
-l experimentally in several ways well known to those skilled in the art.
~" ' ' - ~ .
It has been found that the critical surface tension for pure
- metals (measured in a vacum) is very high, about 1,000 - 4,000 dyne/cm.
Under the conditions ruling in the coating of a base, e.g. paper,
with an extruded p]astic film it has however been found that the
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103~Z46
critical surface tension is considerably lower in the case of a
polished metal roller, namely about 100 dyne/cm. By wetting a
brightly polished cooling roller with water the critical surface
tension of the roller can be reduced to the critical surface
tension of water which is about 73 dyne/cm, because the extruded
plastic coating does not come into contact with the metal roller
;~ but only with the water film covering the roller.
If a polytetrafluoroethylene coat, i.e. the material
sold under the trade name "Teflon", is applied to the pressing
roller, then the critical surface tension of the roller is re-
duced to about 17 dyne/cm, i.e. the critical surface tension of
the Teflon material.
The plastic material used as a coating usually has a cri-
tical sur-Eace tension of about 30 dyne/cm (polyethylene has a cri-
tical tension between 29 and 32 dyne/cm depending on moisture,
temperature and other conditions prevailing during the extrusion).
` By adding ethyl alcohol as a surface-tension reducing
~r agent to water, the surface tension can be reduced as shown by
`~ the table below, which means that the critical surface tension
can be adjusted as desired by adding the substances mentioned,
a fact which - as shown by the description below - is very
~ important.
- The arrangement shown schematically in Fig. 1 has a supply
roll 1 with a strip 2 of paper or paper coated with aluminium foil,
- to which a plastic coating is to be applied. The plastic coat 3,
which consists of polyethylene, is - while still in the
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103~246
~olten stnte - a}-~lie~ to the strip ~ by means of an e-xtrusion
press 4 in which the plastic ~terial is heated to melting and
extruded throu~h a longish, cleft-like valve 5 to form a thin
film 3. The strip 2 coated with the p~astic film 3 is fed in
between two cooperating pressing rolls 6 and 7, of which roll 6
is cooled by a cooling agent, preferably water, which is caused
to circulate within the roll. By means of a spraying arrange-
ment 8, a llquid in finely divided form is sprayed on to the
surface of the roll 6 care being taken that the whole surface
of the roll is covered by a liquid layer preferably of uniform
thickness. The composition of the said liquid is discussed in
detail below and the liquid need not necessarily be applied by
spraying but good results can also be obtained by introducing the
- liquid in other ways, for example by dipping the roller or by
-- 15 applying with a moistened guiding roller. The finished coating
-~ - material is indicated in ~ig. 1 by 11.
'- ' ' ~ ' ' ' ' ; - :
When the plastic coating 3, stabilised by cooling, has passed
- - the cooling roll 6, the contact between the surface of the cooling
~ ~ .
- roll and the plastic coat 3 must be interrupted and, because a
certain amount of sticking of the plastic coat to the cooling roll
always occurs, a tension arises at right angles to the surface of
- the coating material 11. Since the coating material if it consists,
for example, of fibrous material or of fibrous material covered with
aluminium foil, has its lowest tensile strength at right angles to
25 - the surface of the material (the so-callec Z-direction~, tears in
- the material, i.e. so-called cleavage phenomena (delamination)9 are
. liable to occur when the coating material has to be pulled off from
the cooling roll. In general, clea~age does not occur in the
boundary layer between plastic coat and paper, but this cleavage
occurs most fre~uently in the paFer layer close to the plastic coat
as shown by ~ig. 3 which is a 200-fold enlargement of a cross-
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103~Z4~;
sectioll of a delan~ina~cd coating ~tcria].. In Fie. 3 the plastic
coat i5 shown by 12, the fi.brous material by 14 and the delaminated
zone by 13. In order to avoid delamination it is necessary thus
; eithcr to use paper material with such a high Z-strength (tensile
streneth in the Z,direction) that, in spite of the stresses that
arise in the separation of the coating 11 from the cooling roll 6,
,
no delamination occurs, or to ensure that the adhesion between the
. . . cooling roll 6 and the coating material 11 is as small as possible.
,
As-it lS usually difficult to determine the quality and the Z,strength
of the paper, the adhesion between the plastic material and the
cooling roll 6 must be reduced, which can be done in the way shown
by the invention.
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In Fig. 2 a diagram is reproduced in which the horizontal
.:
.: ~axis shows on a linear scale the adhesion between cooling roll and
:~ ". 15 plastic coat and the vertical axis, expressed in dyne/cm, the
- . - critical surface tension. In addition, the dlagram shows a.Curve
. indicated by the Pe which illustrates the adhesion between a
- - ; -
polyethylene coating and a cooling roll with cooling roll coatings
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having various critical surface tensions.
. 20 The horizontal axis has no subdivisions in absolute numbers
; - . because the adhesion depends on several factors, for example the
diameter of the roll and the width of the strip, but the subdivision
.
~ . is in relative numbers 0-10.
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. - ~ As can be`seer.from Fig. 2, the adhesion is greatest (10)
when the cooling roll consists of a dry, polished steel roll with a
; critical surface tension that should here be 100 dyne/cm. If the
: roll is wetted with water, then the critical surface tension
. becom0s about 73 dyne/cm and the a &esion ;s reduced to 7. ~y the .-
.
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103~46
use of a "Teflon" coated cooling roll the critical surface tension
can be still further reduced even to about 17 dyne/cm and, as shown
by the diagram, the adhesion becomes about 5, which must mean that
only a small reduction of the adhesion has occurred although the
critical surface tension has been reduced from 73 to 17. By
looking at the experimentally produced diagram we see that the
smallest possible adhesion, about 3.5, is obtained if the surface
of the cooling roll has a critical surface tension of about
30 dyne/cm, i.e. the same critical surface tension as the poly-
ethylene material.
As already mentioned, the cooling roll is wetted with
a liquid the critical surface tension of which is regulated by
mixing ethyl alcohol in suitable proportions with water. Since
the coating material is to be used for the packing of foodstuffs,
- ethyl alcohol is a satisfactory additive and from the table below
it can be seen that an admixture of about 35% ethylalcohol with
- water gives a surface tension of about 30 dyne/cm.
Water Ethyl alcohol (b.p. 78.5) Critical Surface Tension
100% 0% 72 dyne/cm
2090% 10% 46 "
- 75% 25~ 34 "
5~% 50% 27 "
0~ 100~ 21 "
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lU;~Z46
If ~i miYturc if fcd to the cooling roll 6 in a uniform
nnd un;n~crrupt~d laycr consisting Or 6~ % ~ater and ~5 % ethyl
alcohol, the adhcsion between the cooling roll 6 and the plastic
coat of the coating ~terial 11 becomes as small as possible
(3.5 according to diagram in Fig. 2), and a delamination can be
..
prevented. A greatly enlarged (100-fold) cross-section of a
coating material that had been treated in accordance with the
above 1nstructions, is shown in Fig. 4 where the plastic coat is
indicated by 12 and the fibrous layer by 13 and it oan be seen
from the figure that no delamination has occured.
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- A further advantage of the method of the invention is that
the plastic coat compared with the corresponding surface structure
of a plastic of coat a coating material produced in the traditional
~- - way, has a very uniform surface structure.
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- 15 ~Thus in Fig. 5 a greatly enlarged picture of the surface layer
" . , . , . -, - .. , . :... ," - . . . , . ...... ~ . . . - -
of a plastic coat that has been treated with a dry,.brightly polished
- - cooling roll is reproduced. Astne Figure shows, the surface is full
o~ p1tS which, when they become deep enough, produce holes in the
plastic coat (so-called pin holes). In order to avoid the formation
` 20 of "pinholes", which i~ very important if for example the coating
. . .
~ - --material is to be used for the packing of sterilised goods, the
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thickness of the plastic coat must be great enough to ensure that
the depth of the pits formed in the surface coat do not exceed the
'7 ' ' thickness of the plastic layer. A dimensioning of the plastic
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~ 25 coating without having to take into account the pits formed should
.~ - . .
~ therefore result in considerable commercial gains.
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.
As shown by Fig. 6, which shows exactly the same material
that was treated in accordance with the invention, the surface
structure of the plastic coat which has been treated with a cooling
.
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1038;~4~;
roll to ~liC~I a liquid laycr consis~illg of ~/ater and ethyl
alcohol has becn applied, is considerably more uniform and the
pits that can be seen in Fiz. 5 hardly occur at all. ~ne reason
that the surface structure has been so considerably improved is
not entirely clear but everythine indicates that the ethyl alcohol,
which has a boiling point of 78.5C, is brought to boiling by the
hot plastic coating the surface temperature of which is in general
above 100C when it is brought into contact with the cooling roll.
Cwing to the fact that the ethyl alcohol is brought to boiling a
thin skin of vapour ls formed around the coollng roll, which
contribut-s to the uniform surface structure.
As can be seen from the above, several important advantages
are obtained by means of the method of treatment of the invention
in comparison with the previously known technique because in the
~_- i15- - - first place a reduction of the tension in the Z,direction of the
- coating materlal is achieved and hence a diminished risk of
delamination ànd, ln the second place, a very uniform surface
structure of the plastic coating from the coating material is
achieved which frequently means that the thickness of the plastic
- 20 coat can be reduced because, with-the previous dimensioning of the
plastic coating the risk of so-called pin-hole formation had to be
taken into account.
.
The wetting liquid gives rise, of course, in comparision
with previously known techniques, additional costs but it has proved
- Z5- possible to collect the ~ eater part of the ethyl alcohol for reuse.
- ~ence the costs of the wetting liquid in comparison with the
advantanges achieved are not important.
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