Note: Descriptions are shown in the official language in which they were submitted.
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. -THE-PRIOR ART : : ~ -
In the last years it has become common to use paper sheets :
coupled to pla~tic film, al80 Icnown aH "plasticized paper, " fGr packaging
variou~ article~ . .
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Such plasticed papers have been obtained by coupling, in general
under pressure, a paper sheet to a film of ~;~olymeric material, after treating
the paper sheet with adhesives, or one or both of the components with
electrical discharges of the "corona" type, for promoting their mutual
adhesion.
The process i~ rather complicated and requires separation of the
forming equipment both for the paper sheet and the polymeric film. Moreover
due to the extreme difficulty of obtaining by ext-rusion films of polymers with
a high molecular weight, said process is limited to the use of polymers with
a low molecular weight which yield however film~ of poor mechanical
characteristics and scant "barrier" properties.
The process most comrrionly used by the Prior Art consists in
conveying the paper sheet, treated superficially with a suitable adhesive, to
the head of a polymer film extruder which covers the passing paper sheet with
the polymer film which is made to adhere definitively by compression. The
device illustrated on Figure 1 of the accompanying drawing exemplifies said
process. In this drawing in (1) is represented the paper sheet feeding reel,
~ (2) is the adhesive containing vat, (3) is the dryer, (4) represents the extruder .,
;~l (5) the polymeric film, (6) the rolls for the compression and cooling of the .,
coupled product, and in ~7) is shown the winding up reel for.the finished
, I product. ;'
Such a process offer~ considerable disadvantages due to the low
extrusion speed of the film and to the relative ease with which the latter
breaks leaving the p,aper uncovered, due to the thickness of the film.
` 25 Said drawbacks are increased whenever it is the question of pre-,~ paring multi-layer coupled product~, for instance socalled "sandwich" coup-
ling~ in which the polymer film 1~ ~closed between two pap-r sheet~.
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THE PRESENT INVENTION
An object of this invention is to provide a rapid and cheap process
for preparing "plasticized paper" comprising cellulose paper coupled to ther-
moplastic polymer films and having characteristics superior to those of the
"plasticized papers" known heretofore.
This and other objects are accomplished by this invention in
accordance with which the thermoplastic polymeric material is used in the
form of fibrils dispersed in an aqueous medium.
By the term fibrils as used herein fibers (or fiber - like products)
having a length comprised between 1 and 50 mm, an average (apparent)
diameter ranging from l to 400 micron and a surface area (specific surface)
larger than 1 m /g, are meant.
Said fibrils, which are in the art, may be prepared according to
various different methods and are generally used in the manufacture of -
synthetic paper. A process for preparing fibrils of polyolefinic material i9 :
described, f. i., in Italian Patent No. 947, 919, of Montedison.
According to that process, a solution of the olein polymer is
extruded at a temperature above the boiling temperature of the solvent under
normal conditions, and under autogenous pressure, or at pressure greater
than the autogenous pressure, into a zone of lower pressure, and is then hit
in an at least expanded state by a jet of high speed fluid which is at a
temperature lower than the temperature of the solution, and has an agled
direction with respect to the direction of extrusion of the solution.
Other methods ior obtaining fibrils of synthetic polymer, suited
for manufacturing the coupled sheets of the present invention, are disclosed,
I.i., in U.S. Patents No. 2, 999, 788 and No. 3, 40Z, 231, in British Patent No.
1, 262, 531 and in German Patent Publications DT-OS No. 1, 951, 576. 5 and
D A S No. 1, 290, 040. ~-
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The proce~s of the invention thus .comprise~. following baslc operat
ions carried.out in the order stated:
(a) formation of a panel comprising at least two super-
impoded fibrous layers, each consisting, at least pre-
dominantly, respectively of cellulosic fibers and of fibrils
of synthetic thermopla~tic polymers, obtained by success-
ive stratification of aqueou~ pulp~ or dispersions of such
fibers or fibrils;
(b) reduction of the content in water of the panel up to quantities
comprised between 30% and 80% by weight of water, by
; su.ction and/or compression;
(c) drying the panel until attaining a content in humidity of le~
. than 0 . 5% by weight; and
(d) heating the panel up to a temperature greater than the soften-
ing temperature of the thermoplastic polymer, under pressure.
Between operation (b) and operation (c) there may be inserted a
cold compression step for the panel at pre~sure~ comprised between 50 and
150 kg/sq. cm, which compression may be carried out"for instance between
"hunlid" roll~ or presse~ in those cases in which it ia wished to obtain more
compact papers and lower moisture contents before the drying.
For the preparation of the flbrous panel in step (a), there may be
. used in the form of pulp or a~ueous dispersion, fibril~ made of any synthetic
theremoplastic polymer with a softening temperature not exceeding Z50 C.
i Examples of suitable theremoplastic polymers are, in particular, polyolefins,~
especially low and high density polyethylene, polypropylene prevailingly
; consi~ting of isotactic macromolecules, ethylene/propylène copolymers with
a predominant content of propyl~e, a~ well a~ polyarnides such as nylon 6,
vinyl polymer~ such as polyvinyl chloride and polyvinyl acetate, polyacrylo-
. . nitrile and the polyoxymethylenes.
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Whellever pos9ible, and especially in order to improve the look-
through and the penetration of the plastic layer into the cellulose without re- ¦
.; ducing the "barrier" property, it is prèferred to use mixtures of fibrils of
the same polymer but with different thermoplastic characteristics; for instance
in case of polyethylene fibrils, it i8 preferred to use an aqueous di6persion
consisting of high-density polyethylene fibril~ mixed with low-density
polyethylene fibrils.
Aqueous di~per~ons (or pulp9) of different thermoplastic
polymer~ may in any case be used, provided said polymers are compatible
with each other and have sufficiently close softening temperatures.
In the above-mentioned disper~ions there may also be present
cellulosic fibers in quantities not exceeding 10% by weight on the total weight
of the fibers.
In order to achieve in the finished product the greatest possible
coherence between the cellulose fiber layer and the layer of the thermo-
plastic polymer, it has proved to be quite effective to use, in the panel pre-
'i paration step (a), thermopla~tic fibril~ with si~e~ distributed within rather
,1 wide intervals, preferably comprised between 0. 5 and 5 mm with regard to
their length and between 3 micron and 60 micron as far as the diameter is
concerned .
; Further preferred conditions to be applied in the preparation of the
fibrous panel in operation or step (a) consist in the use of pulps or aqueous
l dispersions of thermoplastic fibrils having a concentration in fibrils equàl to
;' 1. 5 - 2, 5 times the concentration of the cellulosic fibers in the agueou6
dispersions containing the cellulo~ic fibers at least predominantly.
s' In this way on the contact ~urfaces between the layers forming the
panel the possibility for the fibers of each layer to penetrate in the fiber6 ofthe other layer ia increa~ed, with a considerable advantage for the mutual
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adhesion between the thermoplastic phase and the cellulosic phase in the end
product.
The process, on the whole, is easily achieved with the devices
commonly used in paper mill9, devices which comprise cellulose pulp beat-
S ing machines and machines for the forming, drying and calandering of paper
sheets .
The preparation of the multi-layer fibrous panel, according to
step (a), may be carried out, for instance, by the ~uccessive deposition of
the corresponding aqueous dispersions of the fibers on a filtering section,
or by gathering on a common conveyor felt the fibrous pulps directly drawn
from the respective aqueous dispersions by curved or flat table forming
machines arranged in ~eries, according to processes known in the paper
manufacturing art.
Step (b) for the elimination qf the excess water from the panel may
be achieved by passing the panel over suction boxes and/or between pressing
rollers. This operation promotes the phenomenon of the interpenetration
of the two layèrs, made po~sible by the high content in water of the pulps,
and facilitated by the disuniformity of sizes of the fibers, especially of the .
.l thermoplastic fibrila. ' ..
The drying of the panel in step (c) is conveniently carried out
between cylinders maintained at increasing temperatures, but obviously lower
: than the softening temperature of the thermoplastic polymer present in thepanel, and this drying must bring the content in water of the panel down to
not more tha 0. 5% by weight, in order to avoid the formation of vapour
2S bubble~ inside the panel, during the 3ubsequent step (d) heating at high tem-
perature. This latter operation, carried out under pressure, preferably
compriaed between 30 and 100 kg/sq. cm, may be carried out between the
rolls of a glazing machine~ and causes the melting of the thermoplastic
iibril3 with tb orm3tion of a compact layer adhering to the cellDlollic layer. ¦
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If necessary, after the coupling achieved by this
latter operation, the panel may be subjected, after cooling down,
to operations for the superficial treatment of the cellulosic
layer, such as calandering, coating glueing, etc. operations that
are generally used for sheets of cellulose paper.
The pulps or aqueous dispersions, both of the cellu-
losic fibers and of the thermoplastic fibrils, used for the pre- -
paration of the starting panel~ may contain additives such as
dyestuffs, pigments, inorganic fillers, antimildew agents, dis-
persants, etc. The dispersions of cellulosic fibers may also
contain fibers of a different nature, in quantities not ex~eeding
10% by weight with respect to the cellulose material, such as for
instance rayon fibers, or fibers of cellulose acetate, nylon,
polyvinyl alcohol and the likes.
It is essential for the process that the fi~rils of
thermoplastic material be thoroughly and homogeneously dispersed ~-
in water so as to form homogeneous and uniform layers. For this
purpose it is necessary, in general, to add, auring their mixing
with water, disperslng agents such as polyvinyl alcohol, anionic
,20 surfactan~s, urea resins, etc., or to submit the fibrils, before
their use, to a superficial modification which will make them
hydrodispersable. Such a modification may consist in coating the ~-
fibrils with a dispersant, by dipping them into an aqueous solution
of the dispersant and by successively dr~ing them.
; Such a method, wherein dispersants are used which
consist of polyvinyl alcohol/aliphatic aldehydes Cl - C6 conden- -
sates; is described in U.S. Patent No. 4,002,796 (Luciano Baldi et
al) issued January 11, 1977.
The process according to this invention allows to
'30~ obtain sheets of plasticized paper in which the resistance to
~, breakaway or "frilling" of the component layers is practically
indeterminable in as much as this reslstance is greater than the
j tearing resistance of the sheets themselves. A considerable
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a ~nta~e of this proc~ss consists in the fact that thcre
a~ no limits to the speed of preparation of the sheets, since
the resistance of thc cellulose support is never appreciably
influenced by the presence of the thermoplastic material in any
of the processing phases. It is thus possible to use the same
cellulose compositions as well as the same operational speeds
normally used for the preparation of sheets of common paper,
speeds that are by far superior to those usable in the classical
coupling or bonding processes.
By the process according to this invention, it is
possible to prepare "sandwiched" couplings in which, for instance,
the thermoplastic polymeric material in the form of a continuous
film is enclosed between two or more sheets of paper or cellulosic
cardboards.
In drawings which illustrate embodiments of this
invention, Figure 2 represents a ~omplete diagram of the pre- -
; paration of a-binary coupled sheet according to the invention.
Figure 3 is-a complete diagram for the preparation
of a ternary cellulose/thermoplastic material/cellulose coupling.
In the diagram of Figure 2, there are shown the
curved forming boxes (1) and (2~ which contain, respectively, the
cellulose fiber pulps and the thermoplastic polymer fibril pulps;
the con~eyor felt (10) on which the cellulose fiber layer adheres,
and, successively, by superimposition, the layer of thermoplastic
fibrils; the suc~ion boxes (3); "wet" presses (4); the sets of
drying cylinders (5); the heated "glazing" unit (6); the cooling
rolls (~); the take-up reel for the coupled sheet (8), and the
conveyor felts (9). In this diagram the position of the forming
boxes may be inverted still obtaining the same results without
:30 modifying the arrangement of the remaining equipment.
In the daigram of Figure 3 are represented: the
curved forming boxes (l) and ~2), containing the cellulose fiber
pulp; the flat table with inflow box (3),
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containing the fibrils of thermoplastic material, the suction
boxes (4); the "wet" presses (5); the drying set (6); the
heated "glazing" unit (7); the cold rollers (8); the take-up
reel (9) and the conveyor or carrier felts (10).
The following examples are given to further
illustrate the invention and are not intended as limiting.
In all of such examples, olefin polymer fibrils
were utilized, prepared according to the technology illustrated
in the above mentioned Italian Patent No. 947,919.
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EXAMPLE 1
Preparation of polyolefin fibrils
A 50 liter autoclave, provided with heating chamber and stirrer,
was fed with 2. 5 kg of high den~ity polyethylene (M. I. =5, Melting point =
135 C) and 1. 5 kg of kaolin, along with 35 liters of n-hexane. The mixture wa-S heated under the following conditions:
~: temperature = 180 C
total pressure (comprising an overpressure of
nitrogen) = 15 kg/sq. cm.
until a ~olution of the polymer in the n-hexane was obtained.
Under such conditions, the mixture was ejected to the atmosphere,
through a circular noz~le having 2 mm diameter, and was made to collide, at
about 1 mm distance from the nozzle outlet, with a dry saturated steam jet,
coming from a nozzle of 4 mm diameter and arranged at about 85 to the
direction of ejection of the solution, at an impact speed of the steam of about
l 15 470 m/sec. The fibrous product thus obtained was composed of individual
J fibrils having length comprised between 0. 5~ and 5 mm, an average diameter
i of between 15 and 45 micron, a surface area of 5 m ~g and containing abo~dt: :
30% by weight of kaolm.
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~l In two separate vats were prepared two pulps respectively consistt-
ing of
- an aqueous mixture consisting of~ 10 g/lt of cellulose with a re-
fining degree Sl~ = 35; 16% by weight of kaolin and 2% by weight
of ~Aquapel" ( a glue for cellulo~e fibers);
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- an aqueous mixture consi6ting of: 15 g/lt of the poly-
ethylene fibrils prepared a8 described above and which had
been precoated with 0. 05% by weight of a dispersant consist^
ing of a polyvinyl alcohol/butyraldehyde condensate (4. 2 aldehyde
groups/100 monomeric vinyl unitsJby dipping in an aqueous
solution of the dispersant and subsequent drying.
The two pulp9 were conveyed to the corresponding curved forming
boxes, obtaining at the rate of 25 mt/min. a panel consisting of one layer of
cellulose fiber pulp about 2 mm thick, and, superimposed on it, a layer of
polyethylene fibril pulp about 1 mm thick.
After passing over 3 suction boxes arranged in series, the
panel showed a content in water of 80% by weight. Through two wet presses,
exerting a pressure of 50 kg/sq. cm, the panel was then conveyed towards a
battery of 17 drying cylinders of which the firstl¦6 were maintained at at
. 15 temperature increasing from 90C to 110C, and caused drying of the panel
to a residual humidity of about 0. 2Clo by weight, while the ~7th cylinder was
maintained at a temperature of 125 C which induced an incipient melting of
the outer polyethylene fibrils. :
Thereafter, the panel was passed between "glazer" rolls that were
maintained at a temperature of 150 C and exerted a pressl~re of 80 kg/sq. cm,
wherefore there occurred th~ transofrmation of the layer of fibrils into a
translucid and compact polyethylene film of about 20 micron thickness ano
which was adhered to the cellulose sheet. The characteristics of the coupled
Z S ¦¦ p roduct ar umm~ ri~ed in T-ble 1.
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EXAMPLE Z
A binary coupling wa~ prepared by using, for the cellulose fiber
layer, a cellulosic pulp similar to the one described in Example l, and for
the thermoplastic material, an aqueous mixture of uncharged high density
polyethylene fibril~ ~M.I. = 1.2, melting point = 135 C) with a fibril length
. comprised between 1. 5 and 3. S mm and a fibril diameter between 15 and 40
and surface area of 6 m2/g. The concentration in f~bril~ was 20 g/lt and
- the content of polyvinyl alcohol as a dispersant was 1% by weight.
'. The procedures and the equipment for the formation of the two-
layer panel are similar to those de~cribed in Example 1, except that the first
16 cyclinders for the drying were maintained at temperatures increasing from
70 to 90C, while the seventeenth cylinder attained a temperature of 115C.
The temperature of the cylinders of the glazing unit was 175 C. The
cnarsctcriotic f thc coupled product are recordod IA Table l.
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EXAMPLE 3
A binary coupled sheet was prepared by starting from a mixture
of cellulosic fiber~ similar to that in Example 1, and from an aqueou~ dis-
persion of polypropylene fibers (isotacticity index = 92; M.I. at 260C = 10),
melting point = 17 0C) having a diameter comprised between 15 and 3~ micron
a length of between 1 and 4 mm, and a surface area of 7 m2/g, and which had
" been coated superficially like the polyethylene fibrils of Example 1. The
concentration of the polypropylene fibers in the aqueous dispersion was Z0 g/lt .
By using two curved-table forming boxes, arranged in series, the
first containing the dispersion of polypropylene fibrils, the second containing
the cellulose fiber dispersion, on a conveyor net running at 30 mt/min.,
there wa~ prepared a panel consiating of one ground layer of polypropylene
fibrils, about I mm thick, and of a superimposed layer of cellulose fibers
; ~, about 2 mm thick.
Thi~ panel was passed over 3 suction boxes where the content
'il lS in water of the panel was reduced to 75%; it was then passed over "wet"
presses operating at 50 kg/sq. cm and finally was passed on 17 drying cylind-
! ers or rolls of which the first 16 were operating at between 60 and 130C, ~'
while the-seventeenth operated at 165C. After the passage between the rolls
j ~ ~ of a glazing unit, kept at 198C anid exerting a pressure of 60 kg/sq.cm, the
thus obtained coupled sheet was cooled down and wound up on reels. Its
characteri~tic~ are given in Table I. . :
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EXAMPLE 4
By means of a continuous coupling machine of the type illustrated
in Figure 3 of the drawing, there was prepared a "sandwich" type coupled
product consisting of two outer cellulose paper sheets and of an inner low
density polyethylene film.
For this purpose there were used two pulps re~pectively consist-
ing of:
- an aqueous mixture of cellulose fibers having a concentration
in fibers of 10 grams/liter, and containing 18% by weight of
kaolin and 2% by weight of "Aquapel ";
- an aqueous mixture containing 20 g/liter of low-density poly-
ethylene fibrils (M.I. = 5, melting point = 115C) with a
diameter comprised between 10 and 40 micron, a length of
between 1. 5 and 5 mm, a surface area of 6 mZtg and coated
with 0. 05% by weight of the same dispersant as in Example I.
The mixture of cellulose fibers were fed to the two curved-table
forming boxes, while the polyethylene fibril mixture was fed to the flat-table
with in-flow bo:c.
The triple-layer panel:cellulose/polyethylene/cellulo~e, which
after passing over the suction boxes and between the "wet" presses exerting
il 2~ a pressure of 50 kg/sq. cm, was then dried by pas~ing it over 7 drying
rollers or cylinders, maintained at growing temperatures increasing from
~0 to 90 C until the moisture content of the "sandwich" was 0. 1% by weight.
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The melting of the intermediate layer of polyethylene fibrils was
carried out successively by pa~sing the panel between the cylinders or
rolllers of a glazing device, which rollers were maintained at 150C and
exerted a pressure of 75 kg/sq. cm.
After cooling, the coupled sheet was wound on reel~ and atored.
Its characteristic~ are reported in Table I.
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EXAMPLE 5
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Following a traditional method of preparation, carried out on a
~ 10 device similar to that illu~trated in Figure l of the drawing, a binary coupl-
; ' ing was prepared starting from a sheet of packing paper filled with 16% by
weight of kaolin, and by low-density polyethylene granules having a M. I. = 8.
The paper sheet was fir~t impregnated with an adhesive consisting
of a 0. 2% by weight solution of polyisocyanate in methylene chloride, and then
wa~ dried and conveyed at a rate of 100 mt/min. to the head of a wormscrew
extruder loaded with polyethylene granules, where it received on its surface
the semi-molten polyethylene film, in the extrusion phase. ;
The coupled product thus obtained was passed between two
cylinders exerting a pressure of 55 kg/sq. cm, one of the c~r1inders being
~,~ rubberized while the other o~e~was chromium-plated and serving to cool the
,1 polymer layer. The sheet wa~ then gathered on reels and ~tored. The
11 propl~rtil o be collplod ohoee are ~hl~wn in Tablc I.
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