Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION ~ ~
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This invention relates to laminar pellels of
crystalline ethylene terephthalate polymer film and to
improved processes for the solid-phase polymerization and
drying of poly(ethylene terephthalate), hereinafter PET,
and more particularly relates to the solid-phase polymeri-
zation of PET waste materials.
Considerable quantities of waste are accumulated
during the manufacture of PET film. For instance, startup -~;
waste, bead and slitter trim, and reject film may comprise
up to 50% of a biaxially oriented, PET film production run.
The industry has proposed a variety of methods for re-
claiming or recycling these wastes to improve process
economics.
Linear polyesters stored under atmospheric condi-
tions for several days absorb up to 0.4% or more of their
weight of water. Upon remelting such polymer for its
recovery or reuse, this absorbed water can cause a loss of -
up to 20% of its initial intrinsic viscosity (i.e., a
substantial loss in molecular weight). Accordingly, most
commercially acceptable processes for recovery or reuse
require that the polymer be dried or be further polymerized,
or both, before melting to prevent the viscosity from
falling to below acceptable levels for fiber or film
formation.
Because the rate of drying and polymerization
depends upon the rate of diffusion from the bulk of the
polymer of volatile substances, specifically, absorbed
water, and glycol and water liberted from the condensation
reaction as the polyester is further polymerized, it has
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been the practice to grind the film or other polyester :~
material to a fine state. Classically, in U.S. Patent
2 503 251, Edwards et al., which teaches the necessity for
drying to maintain viscosity upon melting, the polymer is
ground to a fine powder. More recently, in U. S. Patent
3 657 388 r Schweitzer et al., it is taught to compress the ~ :
polymer in a "powdery or very fine-grained" state into
particles. Processes for converting the polymer into these
finely divided states are costly in energy consumption and ...
can cause unwanted side reactions to produce color and the
like in polymers such as polyesters. The present invention ~ .
avoids the foregoing and enables the conversion of low bulk - ::
density waste crystalline polyester film into a conveniently
handled state, with a minimum sacrifice in rates of drying,
of polymerization in the solid state, and of dissolution ;~
times in molten polymer and in hot glycol-monomer systems
for glycolysis to monomer.
U. S. Patent 3 767 601 to Knox discloses a prom- .
ising method for reclaiming general-purpose PET film waste,
20 typically having an intrinsic viscosity of about 0.50 to -
0.56, by comminuting the waste to flake form and then sub-
jecting the flake to solid-phase polymerization, in the . ~.
presence of a scavenging gas, to increase the PET intrinsic
viscosity. The resulting waste can be reprocessed by melt
extrusion to make products requiring the properties asso-
ciated with high molecular weight PET. For instance r re- : .
claimed PET having an intrinsic viscosity of slightly above .
0.70 can be used to make a heat-sealable, heat-shrinkable .
film.
It has been found, however, that thin-gauge PET .. -~;.
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flake has such a low bulk density, typically in the range of
2.5 to 5 pounds per cubic foot (40 to 80 kilograms per cubic
meter), that an excessively large reactor or dryer is needed
to achieve practical production levels. Moreover, the low
bulk density may cause material handling problems which limit
process throughput. For example, it is difficult to suffic-
iently agitate a large charge of low-density flake to achieve
uniform exposure to a scavenging gas, such as in the rotary
kiln of Knox or a low pressure environment, and the low-
density flake may clog passageways or become entrained in thescavenging gas stream, thereby clogging venting ports through
which the scavenging gas is discharged.
To overcome these problems, one might melt extrude
the comminuted wastes into solid granules or pellets which
would then be subjected to solid-phase polymerization. But
the reaction rate is limited by the relatively slow diffusion
of ethylene glycol and water by-products to the waste surface
where they evolve. Consequently, the use of larger sized
waste sacrifices reaction rate and, as a practical matter,
increases the plant investment required to achieve a given
production rate.
Thus, there is a need for an improved solid-phase
polymerization process which will give polymerization rates
attainable with finely divided PET wastes, but which will
not present the practical problems associated therewith.
SUMMARY OF THE INVENTION
.
The present invention provides laminar pellets of
waste PET film and a process for heat treating the pellets
for the purposes of drying and solid-phase polymerization
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wherein the pellets are heated to a temperature from about
50C. to below the melting point of the ethylene terephthalate
polymer, most often from 140 to 250C., while volatile sub-
stances are continuously removed. The waste PET is put into
the form of binderless laminar pellets comprising mechanically
interlocked PET flakes.
By "bulk density" is meant the density of an aggre-
gate of discrete items such as flakes or pellets.
By "laminar" is meant that the individual particles
which constitute the pellet have a high surface-to-volume
ratio, such as flakes of film wastes. -~
In practicing the process of the invention, drying
and polymerization rates can be attained approaching that of
the flakes per se without experiencing the problems associated
with processing flake material. The pellets have sufficient
strength to withstand tumbling, mixing, or routine handling
during the process without significant disintegration, and yet
can be readily broken apart after drying or polymerization has
been completed, if desired.
The process is particularly well suited for, and
will hereinafter be described with respect to, drying and
solid-phase polymerization of pellets prepared from waste.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a representation of the porous, laminar
pellet of the present invention.
Figure 2 is a graph illustrating the relationship of
the bulk density of film flake and the pellets of the present
invention vs. the thickness of the film flake.
Figure 3 is a graph illustrating the relationship
between solid-phase polymerization rate and thickness of
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the PET film flake.
DETAILED DESCRIPTION OF THE INVENTION :
Pellets of the invention are laminar, and consist of
mechanically interlocked PET flakes. The pellets do not con-
tain a binder, which would introduce impurities restricting
use of the solid-phase reaction product, but are internally
interlocked so as to be sufficiently cohesive to withstand
tumbling, etc., during a solid-phase polymerization process.
To make the pellets, at least partially crystalline
(i.e., at least 25-30~ crystalline) PET wastes collected
during the manufacture of oriented film are shredded to a
suitable flake size, such as by a rotary cutter, and then fed
to a pellet mill. Flakes which range from about 1.5 mm. to
10 mm. long in each of the two planar dimensions are preferred
for making the pellets. It has been found that PET flake
passing a 3/8-inch (9.5 mm.) screen is particularly well
suited for the preparation of pellets. This flake includes
particles varyingin shape from fiber-like strands to circular
platelets. The pellets of the present invention are illu-
strated in Figure 1. Pellet 10 is typically formed from aplurality of platelets, 11, lla, llb, llc ... lln. The
platelets are mechanically interlocked by slight crimps and
crenulation 12 at the edges, and, in the thinner gauges, by
mechanically binding contact on the surface by creping 13,
which reduces interplanar slippage so that the pellets are
not too readily friable.
While flake bulk density varies with the film
thickness, bulk density of pellets prepared from the flake
remains fairly constant. Typical bulk densities are re-
30 ported in the following Table I for flake and pellets pre- ;~
pared from 0.25, 0.75 and 2.0 mil films.
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It is seen from Table I that the proportionate increase in
bulk density achieved by pelletizing the flake decreases as
film gauge increases. The relation of film gauge to bulk
density is graphically shown in Figure 2, wherein curve I
shows this relationship for film flake from the shredder, and
curve II for pellets of flake formed by a pelletizer. As a
practical consideration, the increase in bulk density obtain-
able with film thicker than 5 mils (125 microns) is generally
so slight that pelletizing of such wastes is not economically
warranted.
Preferred pellet mills have a rotating or stationary
perforated die ring, an internal chamber defined by the die
ring, compression rollers arranged on the inner periphery of
the die ring, and an adjustable rotary or stationary knife
arranged at the outer periphery of the die ring. To make the
pellets, flake of a crystallinity preferably of at least 25-
30% is fed into the internal chamber of the pellet mill where-
in the compression rollers force it outwardly through-the die
ring holes, where the knife cuts the compacted flake into
pellets having the desired length. Suitable pellet mills of
this design are well known in the art. Die rings with holes
of diameter about 3/16-inch to l/4-inch (about 4 to 7 mm.)
are preferred for making pellets of convenient size.
The work performed in the pellet mill, which mechani-
cally interlocks the flakes by slight deformation during forma-
tion results in a temperature rise caused by friction between
the individual flakes and between the flakes and walls of the
die holes. The rise temperature, if above about 180C., will
cause undesirable fusion between the individual ~lakes. ;
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It has been found that pellets prepared from cry-
stalline flake below about 180C., and preferably within the
range of 130 to 160C., in such pellet mills have su-fficient
cohesive integrity to withstand physical handling during the
solid-phase polymerization reaction yet may be easily
crumbled thereafter, if desired. These pellets, typically
having a diameter of about 2 to 7 mm. (about 0.1 to 0.25
inch) and a length of about 6 to 13 mm. tabout .25 to .5
inch), are preferred since a significant degree of fusion
between the flakes may reduce pellet porosity and increase
the effective particle size, thereby reducing the solid-
phase polymerization rate.
As noted in the previous paragraph, the pellets of
the invention have sufficient integrity to survive handling
in operations such as drying and solid-phase polymerization,
yet can be easily broken apart again when desired. The
latter can be accomplished by passing the pellets through fans
in an air transport system. ~eferring again to Figure 2,
curve III shows the bulk density of pellets after one impact
with fan blades, and curve IV shows the same after the third
. . . ..
impact with fan blades, both as a function of -the thickness
of the film flakes from which the pellets are made.
In most situations it is desirable to avoid fusion
between the flakes which constitute the pellet; however, in
some instances, it may be desired to produce pellets with
only a minor degree of fusion, capable of withstanding rough
handling after completion of drying or a solid-phase polymeri-
zation reaction. In that case, a slight degree of fusion at ~ ~
the edges of the pellets can be introduced by preheating the -
flake, increasing the die hole length, or operating the pellet
mill at slightly elevated temperatures. Accordingly, a granu- ~
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lating mill, such as that disclosed in U. S. Patent 3 389 203
to Merges, which operates above the melting point of the poly-
mer, cannot be satisfactorily employed. It will be understood
that fusion decreases porosity of the particles and reduces
the solid-phase polymerization and drying rates.
The solid-phase polymerization reaction is conducted
by feeding the PET waste pellets continuously, or more prefer-
ably as a batch, into a suitable air-tight reaction vessel
maintained at about 175 to 250C., preferably no greater than
220C. The pellets are maintained in the vessel for about 4 to
10 hours until the PET has attained the desired molecular :-
weight increase. During the reaction, the reaction by-pro-
ducts, which include ethylene glycol and water, are continuous-
ly removed to drive the reaction. The reaction by-products are
conveniently removed by passing a dry, inert scavenger gas ..
stream, such as nitrogen or argon, through the reaction chamber ;
or, more preferably, by maintaining the reaction in a vacuum,
typically a pressure level of 3 mm. Hg or less. ~ .
The PET pellets are continuously agitated during
20 drying or solid-phase polymerization processes in order to . :.
uniformly expose the reactor charge to the scavenging gas or .~ -
vacuum. A rotary kiln, tumbling dryer, or similar equipment
may be employed to advantage.
Figure 3 is a graph correlating PET flake polymeri-
zation rate (on the abscissa) to the thickness of the flakes
from which the pellets are formed (on the ordinate) for
solid-phase polymerization reactions conducted at 220C under ~-
vacuum at a pressure of 0.01 to 1 mm Hg, which are typical . ~
reaction conditions. The "polymerization rate", plotted on : .
30 the abscissa, is a measure of the change in polymer intrinsic ; .
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viscosity per hour (~ I.V./hr.), with intrinsic viscosity
being measured in grams per deciliter in a 40/60 parts by
weight solution of tetrachlorethane/phenol at 25C as
described in U.S. Patent 3 627 579 which discloses determi- -
nation of intrinsic viscosity from single values of relative
viscosity. The ordinate is a logarithmic plot of flake thick-
ness from 0.1 to 200 mils (2.5 to 5000 microns). The film
thickness from which the pellets are formed controls the rate
at which ethylene glycol and water can diffuse from the pellet.
From Figure 3, it is readily seen that the PET flake
polymerization rate decreases as flake thickness increases.
When employing the pellets described hereinbefore, however,
the polymerization rate does not depend on the pellet dimen-
sions, as it would with solid pellets. Rather, the pellets
are sufficiently porous that the polymerization rate may
approach that of the individual flakes, especially when the
preferred, nonfused pellets are employed~ For instance,
typical pellets have a polymerization rate within 0.015
intrinsic viscosity units per hour of that exhibited by the
nonpelletized flake.
The pellets have particular utility for increasing ~-
the intrinsic viscosity of thin-gauge film, such as 0.08 to ;
2 mil (2 to 50 microns) film, but can also be employed to ;
advantage with thicker film waste. In a typical case, the
process will be employed to increase the intrinsic viscosity
of waste film from a value of about 0.50 to 0.60 to a value
of about 0.65 to 1.0, or higher, depending on the desired end -
use. For instance, the intrinsic viscosity can be increased
to slightly above 0.65 where the waste is to be reextruded and
3~ unîaxially stretched for use as a strapping film, to slightly
above 0.70 where the waste is to be reextruded and biaxially
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stretched for use as a heat-shrinkable, heat-sealable film,
or to slightly above 0.82 where the film is to be reextruded
and biaxially stretched for use as a film having high pinhole
flex resistance. When the wastes are to be added to virgin
PET, the process can be employed to compensate for decreases
in intrinsic viscosity which occurred during original manu-
facture of the wastes.
As used herein, the term "poly(ethylene terephtha-
late)", PET, means a polymer having the same structure as
that produced by the polyesterification of ethylene glycol
and terephthalic acid. It is believed that this invention
is equally applicable to those polyesters and copolyesters -
similar in structure to PET which are capable of reclamation
by solid-phase polymerization, such as homopolymers and co-
polymers of aromatic dicarboxylic acids, such as terepthalic
acid, isophthalic acid, bibenzoic acid, and naphthalene dicar-
boxylic acids, especially the -2,6-; -2,7- and -1,5- isomers,
with Cl to C10 glycols, such as ethylene glycol, tetramethy-
lene glycol, and cyclohexanedimethanol. The film should be
at least slightly crystalline, preferably at least about
25-30~ to avoid sticking and agglomeration.
EXAMPLE -
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A. Seventy-five-gauge PET film (0.75 mil, 19
microns) having an intrinsic viscosity of about 0.55 was
chipped by a 24-inch (60 cm.) Sprout-Waldron chipper equipped
with a 3/8-inch (9.5 mm.) screen. The rotor was operated at
1200 rpm, and 1700 cfm (4.8 cubic meters per minute) air was -~
used through the screen. The throughput rate was 1300 pounds
per hour (590 kilograms per hour) and the resulting flake bulk
density was 4.8 pounds per cubic foot (76 kilograms per cubic
meter).
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A 3.5-cubic foot (16.8 lbs., 7.6 kg.) charge of the
flake was fed to a Patterson Kelly tumble vacuum dryer. The
dryer was then heated to 220C. under vacuum, at a pressure
of 0.01 to 0.5 mm. Hg, absolute, and the flake was kept in
the dryer for 4 hours after it reached the dryer temperature.
The resulting flake had an intrinsic viscosity of 0.99. By
computation, the polymerization rate was 0.11 intrinsic vis-
cosity units per hour.
B. A portion of the PET flake from the Sprout-
Waldron chipper was fed to a Model cMFs California Pellet
Mill equipped with a die having 1300 3/16-inch (4.75 mm.)
diameter holes l/4-inch (6 mm.) long. The mill operated at
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441 pounds per hour (200 kilograms per hour) and produced
3/16-inch (4.75 mm.) diameter pellets having a length of
1/2-inch (12.7 mm.) at 135C. The pellets readily feed out
from a storage bin, and have significantly less tendency to
clog passageways and venting ports than does the flake from ~ -
which the pellets were made.
A 3.5-cubic foot (91 lbs., 41 kg.) charge of the
pellets was fed to a Patterson Kelly tumble vacuum dryer.
The dryer was then heated to 220C. under vacuum, at a pres-
sure of 0.01 to 0.5 mm. Hg, absolute, and the pellets were
kept in the dryer for 4 hours after reaching dryer temperature.
Resulting pellets had an intrins~c viscosity of 0.95. By com-
putation, the polymerization rate was 0.10, which compares
favourably with the polymerization rate for flake reported
in part A. The pellets remained substantially intact.
C. Polymerized pellets of part B were processed
through a conveying blower. The bulk density was decreased
to 14 pounds per cubic foot (224 kilograms per cubic meter).
After subsequent passes through the blower, the bulk density
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fell to 9.0 pounds per cubic foot (144 kilograms per cubic
meter). The decrease in bulk density illustrated that the
pellets can be readily broken apart, if desired, after com-
pletion of the solid-phase polymerization, by passage through
fans, as shown in Figure 2.
The example illustrates that the pellet polymeriza-
tion rate is comparable to that of the flake. Both the flake
and pellet polymerization rates were slightly below that pre-
dicted by the graph, evidently because of poor temperature
control of the dryer.
Drying, which normally precedes other processes as
mentioned earlier herein, can be carried out either as a
separate operation or integrated with the process of solid-
state polymerization, but at somewhat lower temperature.
Generally, it can be integrated with solid-phase polymeriza-
tion, wherein drying is conducted at temperatures of up to
about 140C. in a dry inert atmosphere at a pressure of 100
mm. Hg or less, or with a stream of a dry, heated inert `
scavenging gas, such as air or nitrogen. The second stage,
polymerization, preferably employs temperatures of the order
of 200 to 220C., but oxygen, as in air, should be excluded.
Table II illustrates typical drying times for film
flakes of various thicknesses, and pellets of two types.
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TABLE II
Times to dry PET from 0.30% H2O
to 0.01% H2O at 140C. in dry
nitrogen stream
Thickness (microns) Time (minutes)
13 2
250 9-3
750 13.4
Solid Pellets(l) 45
Compacted Pellets~2) 9
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(1) The solid pellets were substantially
cylindrical, formed from molton PET
extruded rods l/8-inch diameter,
1/8-inch long (3 mm. by 3 mm.).
(2) The compacted pellets were of film
38 microns thick, and formed according
to the present invention.
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