Note: Descriptions are shown in the official language in which they were submitted.
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. BACKGROUND OF THE INVENTION
.`; (1) Field of the Invention
~: ~
~-' The invention relates to a process for selectively
, recovering thermoplastic polymers, and particularly polyester
';''' polymers from collections of yarns, films, fibers or fabrics,
including dyed polyester fibers, for use in production of new
undyed thermoplastic products, particularly polyester fibers,
-. films and the like. More specifically, the invention relates
~ to a process for selectively recovering polyester polymer
" 10 by means of stripping the dye from dyed polyester fibers,
'?~ subsequently dissolving the polyester fibers, separating the
~;~; solution from any insoluble materials, and thereafter
; recoverying the polymer, without precipitation thereof, by
evaporation of the solvent from the molten polymer.
(2) Prior Art
The consumption of thermoplastic polymers is greater ~ n ten
` billion pounds (4.5 billion kilograms) per year. Some, such as
polyethylene, polypropylene, polyvinylchloride, polystyrene,
polyamides and polyester, surpass the billion pounds per
year rate. The use of many of these relatively expensive
;~ - thermoplastic polymers for snythetic fibers has, notwith-
'~ standing sig~ificant periods of decline, increased
`~` tremendously.
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Concomitantly the world is facing a shortage of
raw materials for thermoplastic polymers; and sophisticated
`` and efficient methods of recycling are needed.
.
- Various methods have been described in the prior
art for separation and/or recovery of thermoplastic polymer,
including polyester polymers, from scrap polymers; and
..
these have included the dissolution of the polymer in
- various solvents; thereafter precipitating and recovering -
the polymer. The objects of such processes were to avoid
polymer degradation and/or to separate from the usable
polymer the degraded polymer and/or monomers as impurities.
The processes were slow and expensive; suitable only for
laboratory usage; and they neither addressed themselves
to nor did they solve fiber separation and dye removal
. . .
problems.
Cur U.S. Patent Number 4,003>880 discloses a process for
;~ recoverying polyester from waste fabrics or fibers by
dye stripping, selectively dissolving the polyester fibers,
removing the undissolved fibers and any other undissolved
impurities from the solution; and thereafter precipitating
polyester out of, and separating the polyester from the
solution.
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~r Whether by prior art processes or by the above
`. described process, the precipitation of polyester out of
solution inevitability brings forth with the precipitated
.... . .. .
;x~ polyester polymer substantial quantities of the solvent
and dye which must be separated from the newly precipitated
polymer by way of decantation and/or filtration (for
.
; .. .
-. gross separation~ as well as evaporation or solvent leaching
or washing (for trace removal). Since the precipitation
.. ~ ..
- step does not eliminate the necessity for evaporation or.-~ 10 equivalent treatment, and complicates rather than simplifies
dye removal, it will be seen that elimination of the
precipitation step in an efficient method of separating
the polyester polymer from the solvent would be a
;~ meritorious advance in the art, and constitutes a primary
~,,", .
object of this invention. :
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,~!`j~,`. It is another advantage of this invention ~hat
~,' the same solvent or solvent system may be used for both
i. dye-stripping and polymer recovery, as well as for fiber
; . .
'.; separation.
;. ~
~c 20 It is also an advantage of this invention that
. . ,~
; removal of solvent from recovered polyester is greatly
simplified and that solvent removal may be conveniently
`:;` combined with a polymerization step wherever needed to
, increase molecular weight.
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SUMMARY OF THE INVENTION
In accordance with the present invention, a
process is provided whereby dye-s~ripping, separation and
i polymer recovery are combined in such a manner that the ~:
~ dye-stripping phase actually constitutes the first step :
of the dissolution and separation phase in that after
completion of the so-called "dye-stripping" phase the
fibers remain saturated with a dye-stripping solvent which
; serves as part of the solvent used for dissolution of the
polyester, so that only one solvent system may be employed.
Briefly, the inventive concept is a process for
. selectively recovering polyester polymer from collections
: -'.
of dyed fibers or fabrics including dyed polyester fibers,
comprising:
(1) contacting collections of yarns, films, fibers,
~ or fabrics, including dyed polyester fibers with a dye-
;. stripping solvent for polyester polymer which is preferably
not a solvent for the remaining constituents at a temperature
.; below which the polyester fibers dissolve and above which
the crystalline lattice of the polyester fibers swell so
as to release the dye, thereby stripping the dye from the
~i polyester fibers;
(2) removing the excess of the dye-containing
: dye-stripping solvent which is not absorbed by the fibers
and fabrics;
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. (3) contacting the fibers (which may contain
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~- residual dye-stripping solvent) with sufficient addition
~` of a primary dissolution solvent under selective dissolution
::.' ' .
. conditions for polyester fibers;
: (4) removing the undissolved fibers or other solid
; impurities from the solution; and
(5) separating the solvent or solvents from the
~ polyester by evaporating the solvent from dissolved and/or
'~t'.~. molten polyester without precipitating the polyester from
t' ~
solution.
DESCRIPTION OF THE PREFEXRED EMBODIMENTS
, For the purposes of this description, solvents
: will be classified as "dye-stripping solvents" and
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~ "primary dissolution solvents". A l'dye-stripping solvent",
~ .
~:` as used herein, is any solvent which swells the crystalline
~ structure of the polyester fiber, at the same time dis-
;`` solving and thereby removing conventional dyes and finishes.
~ It is selective in nature in the sense that it will
r
~;~ dissolve a minimum, if anyJ of polyester and preferably will
l: 20 neither dissolve nor swell other components in the starting
.. t collection of materials. A "primary dissolution solventl'
~. is a solvent whose primary function in the course of this
-~ invention is to dissolve the polyester. It should have the
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characteristic of dissolving a significant amount of
polyester, selectively with respect to other components
in the starting material, at moderate temperatures, all
-~ the while permitting subsequent removal from the polyester
by vaporization. All presently known "primary dissolution
- solvents" are also "dye-stripping" solvents (when employed
at lower temperatures and/or lower concentrations), but
the converse is not necessarily true. Of course it is
-~ preferred, according to this invention, that the dye-
stripping solvent and the primary dissolution solvent
be the same, and such identity is one aspect of this
;.i .
~ invention. If identical solvents are not used, it is
; desirable that they be compatible in the sense that they
; do not functionally interfere with one another and that
they be readily separable or functionally interchangeable,
- as this will permit a much more simplified recovery system.
~ :.
' It is also preferred that solvents employed in
this invention do not significantly degrade or depolymerize
the polyester under conditions required for removal by
evaporation. In addition, the solvents should have the
,.i
characteristlc of being essentially non-solvents for the
other components in a collection of yarns, films, fibers,
fabrics, etc., if such a collection is employed as the
starting material. Of course, whenever solvents are used
.
. together they should be compatible in the sense that they
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do not exylode or react violently.
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Suitable dye-stripping and primary dissolution
solvents include most of the so-called "polyester dye
carriers" which can be easily removed from the fibers
under vacuum at a temperature at which no degradation occurs.
It is well known, for example, that most solutions of the
. .
following compounds will cause an increase in the diameter
of the polyester fiber immersed therein: phenol, meta-
cresol, tetrahydronaphthalene, ortho-phenylphenol, para-
phenylphenol, and such compounds may be employed as either
. .
10 dye-stripping solvents or primary dissolution solvents or
both. Other known solvents and solvent systems for polyester
which may be employed for both dye removal and dissolution
:. ~
include para-chloroanisole, nitrobenzene, acetophenone,
propylene carbonate, dimethyl sulfoxide, 2,6 xylenol,
,
quinoline, trifluoroacetic acid, ortho-chlorophenol and
trichlorophenol. Preferred are polyester solvents and
solvent systems which include compounds having at least one
or more and more commonly two or more aromatic rings in their
.
structure such as biphenyl, diphenyl ether, naphthalene,
' 20 methylnaphthalene, benzophenone, diphenylmethane, para-
dichlorobenzene, acenaphthene, phPnanthrene and similar
compounds. Naphthalene has been found especially suitable
for the practice of this invention for dye-stripping and as
a primary dissolution solvent because the solubility of
polyester in naphthalene is a strong function of ~emperature,
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`' ranging from 0 solubility at 170C. to about 55 percent
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. polyester solubility at 218C. (boiling point of naphthalene).
.
~ Naphthalene is also highly selective in the sense that
~;
although minor amounts of nylon 66(<0.1%) may dissolve,
`.~ it will not, at up to 218C., dissolve most other common
. fibers including acetate, cotton, rayon, wool, silk, flax,
nylon-6, acrylic, glass and metallic fibers, nor will it
dissolve paper, glass or metal scraps.
. -
Polyester solvents and solvent systems other than
naphthalene which are known to be and have been demonstrated
as suitable for dye-stripping and selective dissolution of
polyester, are listed in the following table showing
;.................................................................. .
~ acceptable dissolution conditions only.
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~:` The preliminary dye removal may be accomplished
by any method of immersing, or otherwise intimately
contacting and agitating the fiber or fabric collection
with the dye-stripping solvent, in any manner which removes
aLl or most of the dye concentration in the fiber or fabric
collection. Any one of the following methods may be used
to accomplish the preliminary dye removal. Although the
- starting material is described as "fabric" or "fabric
collection", it should be understood to include "fibers"
; 10 as hereina~ter defined.
1. Contacting a batch of fabric with a large
amount of dye-stripping solvent, which
quantity is large enough to dilute the dye
~ concentration in the fabric to the desired ;,
- level.
- 2. Contacting a batch of fabric with dye-stripping
solvent while the dye-containing solvent is
agitated if desired and continuously removed
and replaced with fresh or relatlvely dye-
free dye-stripping solvent, in sufficient
quantity to reduce the fabric's dye concen-
- tration to the desired level.
..;
3. Contacting a batch of fabric with resh or
relatively dye-free dye-stripping solvent,
using agitation if desired, for a given
batch contact time; thereafter removing sub-
~ stantially all of the dye-containing solvent
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and contacting the fabric with fresh or
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relatively dye-free dye-stripping solvent,
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. with agitation if desired, for some period of
contact time which may differ from the
original or subsequent batch contact times;
~ thereafter repeating as many times as desired
: such dye-stripping solvent addition, fabric
.,.
contacting, and solvent removal, in order to
obtain the desired degree of dye removal from
the fabric. (This method is substantially
~- equivalent to the laboratory Soxhlet extractor.)
4. Fabric or a fabric collection is continuously
.. . .
. moved along a path or conduit~in one direction
.:
while simultaneously being contacted with a
~; dye-stripping solvent, which solvent is more
or less continuously flowing in a direction
: opposite to the movement of the fabric. Fresh
; or relatively dye-free dye-stripping solvent
is added in a manner which maintains a
:
relatively continuous flow of the dye-stripping
solvent, and the dye-containing dye-stripping
.'!' solvent is more or less continuously removed
.I at or near the place where the abric or
- fabric collection is first contacted with the
r ;' dye-stripping solvent.
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5. Fabric or a fabric collection is successively
contacted with dye-stripping solvent in a
q multiplicity of dye-stripping solvent contact
stages, with said contact stages arranged in
such a manner that each subsequent contact
stage reduces the dye concentration in the
fabric collection; especially a counter-
current flow arrangement of contact stages,
in which fresh or relatively dye-free dye-
- 10 stripping solvent is added only to the final
fabric contact stage, with a more or less
equal amount of dye-containing dye-stripping
solvent removed from the final stage and added
to the dye-stripping solvent in the nex~-to- :
final contact stage, such counter-current flow
:;
replenishment continues for as many contact
stages as are used, with the dye-laden dye-
,. stripping solvent removed from the first fabric
contact stage.
.'' :'
Of course, in these dye-stripping solvent contact-
:~;
ing processes, the fabric or fabric collection may be
added to the dye-stripping solvent, or the dye-stripping
,
siolvent may be added to the fabric or fabric collection.
Similarly, of course~ the fabric may be moved through dye-
. stripping sol.vent which is kept more or less in one place;
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or the fabric may be held more or less at one place while
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-: the dye-stripping solvent is moved into contact with and
subsequently removed from the fabric or fabric collection;
. or, both the fabric and dye-stripping solvent may be moved
simultaneously or alternately.
.
:-: For efficient dye-stripping without polymer loss,
,~ the temperature of the dye-stripping solvent during the
dye-stripping phase must be below the temperature at which
there is significant dissolution of the poLyester fibers.
-- 10 However, it is well known among textile dyeing and finishing
experts that most efficient dye-stripping of polyester
fibers will occur at the highest temperature practical
,,~
-. because of swelling of the crystalline lattice of the
- polyester is greatest at the higher temperatures. A
;, significantly lower temperature will decrease swelling of
~ the fiber> and at just above the freezing or solidification
.~ point of the solvent, ~here will be little or no dye-stripping.
- Preferred, therefore, is the highest temperature below
;~ which there is significant dissolution of the polyester.
, .
Apparatus or equipment which may be used for the
preliminary dye removal operation include tanks or vats,
which may be agitated or not agitated, whether open top or
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covered or sealed to hold pressure or vacuum; bowl-type
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washing machines; pressure dyeing apparatus; centrifugal
filters, with or without provisions for solvent rinsing
or continuous or intermittent removal of fabric; continuously
:-~ or intermittently moving conveyor belts passing through
solvent-contacting zones; screw conveyor devices; and
solvent spraying devices.
. .,
-~ When the preliminary dy~e-stripping step is
:.
completed, and the dye-containing dye-stripping solvent is
removed, the, remaining wet fabric or collection of
, 10 fabrics containing residual dye and solvent is contacted
with sufficient additional relatively dye-free primary
solvent under dissolution conditions for the polyester
fibers. Of course, the residual dye-containing solvent-
laden fabric may be added to the primary solvent, or the
;, ~
primary solvent may be added to the fabric. As previously
mentioned, the additional primary dissolution solvent
`,~ may or may not be the same solvent or solvent system as
employed for dye-stripping; but it is preferable to use
`.` the same solvent or solvent system for process efficiency,
r 20 simplicity, and economy. A recycling system is much
, .,
7''' preferred over a non-recycling system; and it might well
,; be essential to the commercial feasibility of the process.
Incompatible solvents or solvent systems would add to the
, complexity and cost of any such recycling.
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When the polyester fibers have dissolved, any
undissolved fabrics are removed from solution for discard
or for subsequent use. The removal may be accomplished by
any known physical separation procedures such as screening,
-~ centrifuging, decanting, filtration or any combination of
these procedures.
... ~
We have found that polyester can be then recovered
- from a molten polyester solution by heating the solution
above the boiling point of the solvent (or solvents),
removing solvent vapors (for subsequent condensation and
reuse), possibly displacing the solvent vapors with another
gas such as an inert gas sweep. The removal of solvent by
atmospheric boiling is usually slow and may require temper-
atures so far above the melting point of the polyester that
: polymer degradation can occur. Atmospheric boiling may
also result in a high level of residual solvent with the
polymer. In a solution composed of 70% naphthalene and
30% polyester, for example, atmospheric boiling at 260
failed to remove at least about 12% of the naphthalene. It
has been found that a much more efficient procedure in the
-' recovery of polyester from most dissolution solvents (and
most particularly from the preferred class oi solvents having
at least one and more commonly two or more of the aromatic
rings in their structure) is to boil vigorously at
atmospheric pressure for a short time using a heat transfer
medium slightly above the melting point of the polyester,
and then sustain this vigorous boiling by applying a vacuum
to the molten solution.
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Of course temperature and pressure interact
.` during the vacuum solvent boil-off. If the vacuum is
` applied too rapidly, very rapid vaporization of solvent
cools the molten polyester solution enough to actually
~ freeze it. If the polyester solution freezes (even only
,: partially), the vaporization process is slowed considerably,
. . ~
~: and the recovered polyester will contain much more residual
.` solvent. The higher the temperature of the molten polyester
.. : solution, the faster the vacuum can be applied for rapid
: 10 solvent removal without freezing. As mentioned before,
however, the higher temperature can promote polyes~er
degradation; and therefore a balance must be maintained
between the solution temperature and the rate of vacuum
, application.
'.' ~
.~ Polyesters and copolyesters known to be useful
~, in the practice of this invention are those derived from
. aromatic dicarboxylic acids such as terephthalic acid and
isophthalic acid and glycols such as ethylene and butylene
glycol. Representative examples include polytethylene
.- 20 terephthalate), poly(trimethylene terephthalate), poly-
(tetramethylene terephthalate), poly(ethylene isophthalate),
. poly(octamethylene terephthalate), poly(decamethylene
- terephthalate), poly(pentamethylene isophthalate), poly-
'~ (tetramethylene isophthalate), poly(hexamethylene isophthalate),
poly(l,4-cyclohexylene terephthalate), and poly(ethylene-
co-tetramethylene terephthalate).
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Unless otherwise indicated, the terms "collections
~ of fibers" and "polyester fibers", as used herein to
~ describe the starting material which is subjected to dye-
- stripping, separation and recovery in accordance with this
invention, includes fibers, filaments, monofilaments, bands,
:
ribbons, tubes, films and other constructions of linear
polyester and includes yarns, threads, fabrics and other
products into which these constructions may be incorporated,
as well as common impurities associated with such products,
new or old.
; ~
` EXAMPLES
The feedstock was first prewashed in naphthalene
~ at 165C.(whenever dye was present in the starting material).
; Polyester feedstock was then dissolved in naphthalene at
- about 30% polyester and at a solution temperature of
about 210C. The solution was then filtered to remove in-
soluble contaminates such as nylon, acrylics, paper, cotton,
... .
` wool, silk, rayon, acetate and metals. The polyester/nath-
thalene solution was then boiled in an oil-heated 3-neck
boiling flask, fitted with a thermometer, an inert gas purge
nozzle, and a short air-cooled condenser connected to a
~,i .
, 2-neck flask, which functions as a naphthalene collector.
; A small purge of inert gas was used to sweep naphthalene
.. . .
vapors into the condenser. Boiling was accomplished in
an oil bath maintained at a temperature of 260C.-270C.
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~ When the rate of boiling declined, the vacuum was slowly
- applied to the apparatus through the second neck of the
'~ naphthalene collector. A vacuum regulator was used to
control the vacuum pull-down rate on the boiling solution
to minimize foaming, splashing, and possible freezing of
the solution. When solvent removal was completed, the
~-~ vacuum was replaced with dry nitrogen and the flask was
s removed from the hot oil. The inert blanket was maintained
, as the polyester cooled and froze. The flask was wrapped
in cloth because expansion of the solidifying polyester
breaks the flask. After cooling was complete, the
,~ polyester produrt was recovered from the broken flask for
~,.
analysis and subsequent spinning to fiber.
'`'`'''
r,:' Using a controlled polyester flake stock having
~ an intrinsic viscosity of 0.6413, the following recovery
..':
~; processes were employed as above using the finishing
vacuum as indicated.
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. As can be seen from Table II, there was no
.~ significant drop in the intrinsic viscosity of the product,
-. and in some cases it appeared that the viscosity was
` increased. Any such increase is believed to have been
` caused by further polymerization occurring during the
vacuum removaI phase.
Mixed polyester waste fabric was subjected to
. the same procedure, with the result that although the
~ initial intrinsic viscosity could not be ascertained,
,~( 10 recovered polymer of comparable fiber viscosity was obtained
'',! which appeared to be reasonably pure based on color
observation. These recovered polymer sam~les were of a
quality which could be subjected to melt spinning into
synthetic fibers. The spinning procedure was to place
'J chunks of recovered polyester in a laboratory autoclave;
)
pressurize several times with dry nitrogen; place a
vacuum (20-40mm Hg) on the autoclave; heat the autoclave
until the polyester melts; bleed off the vacuum with dry
nitrogen; remove the agitator shaft and affix a spinneret
pack; apply dry nitrogen pressure to orm filaments from
the spinneret; and take up bobbins of the fiber on a
Leesona Type 955 winder. Spun bobbins were subsequently
drawn over a hot pin at 105C. at several known draw ratios
and physical properties of the drawn fiber were measured.
Only crude indications of spinnability and drawability
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were obtained due in large part to the lack of denier
. control. The intrinsic viscosity of the recovered prewashed
.: polyester mixed rag feedstock was 0.6867. The spun yarn
:~
:~ (10 filaments with con~entional fiber finish) was
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successfully drawn over a hot pin at 105C., with drawn
: fiber properties as indicated in Table III.
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; TABLE III , .
.-; Draw Drawn Tenacity Elongation
~;~. Ratio Denier (g/d) (%) :
. 10 4.78 85.5 3.88 15.37
., 5.00 83.0 4.33 10.62
5.50 77.5 3.43 7.23
Having thus described our invention and several
'! embodiments thereof, what we desire to claim and secure by
: letters patent is: : -.
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