Note: Descriptions are shown in the official language in which they were submitted.
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Olefin polymer films, particularly ethylene
polymer films, are used in commerce in large volume. By
reason of their volume usage, the art is continuously
seeking (1) olefin polymer films having improved prop-
erties and (2) more economical processes for the manufacture
of olefin polymer films.
The invention is directed to an improved process for
preparing olefin polymer films in which a web of randomly
dispersed olefin polymer fibrils are heated under pressure to
melt the fibrils to form a fused film. The temperature and
the time of pressing are controlled so that a small percentage
of fibrils do not completely melt. The unmelted fibrils in the
fused film reinforce the film and improve its tensile strength.
Thus according to the present invention, there is
provided a process for preparing a film of fused olefin polymer
that is reinforced with unfused fibrils of an olefin polymer
which consists essentially of:
(a) Forming a web of randomly dispersed olefin
polymer fibrilsl
(b) Heating the web of step (a) to a temperature at
which said fibrils melt,
(c) Subjecting the web to pressure throughout step
(b) so as to form a compressed melted film, and
(d) Cooling the melted film of step (c) to form a
self-supporting film;
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the olefin polymer included in said fibrils having a weight
average molecular weight of at least one million; the duration
of heating step (b) being such that a portion of the fibrils
are incompletely melted and retain their fibril identity in
the fused olefin polymer film.
The olefin polymer fibrils employed in the practice
of the invention constitute a recognized class of materials
known and described in the art. Such fibrils are prepared
by precipitating an olefin polymer from an organic solvent
solution thereof under conditions of- high shear. Representative
U. S. patents disclosing the preparation of such fibrils include
the following:
U.S. 2,988,782
U.S. 3,081,519
U.S. 3,341,242
U.S. 3,740,383
U.S. 3,743,272
U.S. 3,808,091
U.S. 3,885,014
U.S. 3,891,499
U.S. 4,010,229
U.S. 4,013,751
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When examined with the naked eye, the olefin polymer
fibrils employed in the invention resemble in appearance natural
fibers such as cotton and staple fibers of solution spun polymers
such as nylon and the like. When examined microscopically,
however, fibrils have grossly irregular shapes, including sey-
ments which appear to be film-like in nature. See for example
Fig. 4 of U.S. 3,891,499. It is believed possible that these
particular physical characteristics may play a role in the
function of the olefin polymer fibrils in the present invention.
The majority of the olefin polymer fibrils employed
in the practice of the invention are samll in size, with the
largest dimension not exceeding about 10 mm. The fibrils
have a large surface to weight ratio of at least about 2 m2/gram
as determined by nitrogen absorption/desorption methods.
The olefin polymer fibrils emplo~ed in the invention
are prepared from an ethylene or propylene polymer having a
weight average molecular weight of at least about 1 mill-ion and
preferably at least about 1.5 million. Such polymers will have
an intrinsic viscosity of at least 3.5, preferably at least 5.0,
and most especially at least 10Ø A listing of suitable olefin
polymers and olefin polymer mixtures is set forth in U.S. 4,013,~51
.
The web of randomly-dispersed olefin polymer fibrils
employed in the invention can be prepared by numerous techniques
known and reported in the art. The preferred method is to
prepare a water-laid web by the techniques known and used in the
paper making arts.
The fused film is prepared by heating the web to a
temperature above the melting point of the olefin polymer and
subjecting the web to pressure to compress the melted polymer
to form a fused film. The pressure required and the duration
of the required heating cycle will depend upon a multiplicity
of factors including the thickness and bulk density of the web,
the melting point of the olefin polymer, and the heat transfer
characteristics of the press surfaces. The proper combination
of temperature and pressure to be employed for a given web in a
particular press can be readily established through a few routine
experiments.
In pressing a given web at a fixed temperature and a
fixed pressure, it is noted that the fibrils at the surface of
the web melt first. With the passage of time, the fibrils in
the interior of the web melt. If the heating is continued for
a sufficient period of time, all of the fibrils melt and a
homogeneous fused film is obtained. At intermediate points in
the heating cycle, it is noted that the film, while fused, will
contain thin fiber-like sections of unmelted fibrils. Film con-
taining such a structure has a tensile strength greater than
solvent cast film prepared from the olefin polymer from which the
fibrils were prepared. Accordingly, the temperature, pressure
and cycle time should be controlled so that the fused film contains
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some incompletely melted fibrils.
A desirable feature of the invention is that the
fused film can be prepared at high rates of speed at low cost
in a continuous process. The random web of fibrils is laid down
continuously on a paper making machine. The web then is passed
over drying rolls and passed through a stack of heated calender
rolls.
The following example i6 set forth to illustrate more
clearly the principles and practice of this invention to those
skilled in the art. Where parts or percentages are referred to,
they are parts or percentages by weight unless otherwise noted~
Example 1
Part A
A lot of fibrils was prepared following the procedure
of Example 1 of U.S. 4,013,751. The ethylene polymer from which
the fibrils were prepared had a weight average molecular weight
of about 1,500,000. The hydrocarbon-wet fibrils were refined
with 91% isopropanol to displace the hydrocarbon. The fibrils
then were filtered and pressed to expel the maximum quantity of
isopropanol.
Part B
Water-laid sheets were prepared from the fibrils of
Part A employing an experimental size paper making machine. The
dried sheets had a basis weight of about 60 lbs. per ream. An
8" x 8" sheet was pressed for 5 minutes in a platen press under
a pressure of 40,000 psig at a plate temperature of 300F (149C).
The resulting film was well fused and generally transparent.
When viewed in strong light, a few fine fibers were noted in the
film. The film had a tensile strength of 4200 psi.
Part C
A hydrocarbon solution was prepared from the ethylene
polymer employed in Part A and a film was cast therefrom. The
film had a tensile strength of 2900 psi.
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