Polyester polymer recovery

Polyester fibers are dissolved and solid linear polyester polymer recovered for use in the production of new linear polyester products, particularly polyester fibers or filaments, by the process of dissolving the fibers in a non-depolymerizing dissolution solvent for polyester characterized by inclusion of carbocyclic rings in its structural formula under dissolution conditions for polyester polymer; thereafter quenching the polyester solution; and separating the solid polyester from the solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for recovering linear thermoplastic 
polymers, particularly polyester polymers, from polyester fibers, 
including collections of polyester yarns, films, fibers or fabrics, for 
use in production of new polyester fibers, films or other products. More 
specifically, the invention relates to a process for recovering linear 
polyester polymer by means of dissolving the fibers; thereafter quenching 
the polyester solution, thereby to precipitate out and recover the linear 
polyester polymer for reuse. 
2. Prior Art 
Various methods have been described in the prior art for the recovery of 
thermoplastic polymer, including polyester polymers, from scrap polymer; 
and these include the dissolution of the polymer in various solvents 
including naphthalene, thereafter precipitating and recovering the 
polymer. Typical of such processes is U.S. Pat. No. 2,762,788, the objects 
of which were to avoid polymer degradation and/or to separate from the 
useable polymer the degraded polymer and/or monomers and oligimers as 
impurities. These processes were slow and expensive; suitable only for 
laboratory usage. 
It will thus be recognized that a satisfactory, rapid, and efficient 
process for recovery of polyester polymer from polyester polymer fibers or 
fabric would be a meritorious advance in the art. It would substantially 
reduce the raw material requirement for the world's largest fiber market. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a totally new process is provided 
for polymer recovery. 
Briefly, the inventive concept is a process for recovering linear polyester 
polymer from polyester fibers or fabrics comprising: 
1. dissolving polyester fibers in a non-depolymerizing dissolution solvent 
for polyester, characterized by inclusion of carbocyclic rings in the 
structural formula of the solvent base, to form a solution; 
2. thereafter, precipitating out the polyester by quenching the solution; 
and, 
3. separating the precipitated polyester from the dissolution solvent (and 
quenching medium, if applicable). 
It is an advantage of this invention that preferred solvent systems are 
employed efficiently and rapidly in the recovery process. 
It is another advantage of this invention that removal of solvent from 
recovered polymer is greatly simplified.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
We have discovered that polyester may be efficiently recovered by quenching 
from any solution of polyester and non-depolymerizing dissolution solvent 
for polyester which is characterized by inclusion of carbocyclic rings, 
preferably 1-3 carbocyclic rings, in the structural formula of the solvent 
base. Solvent selection, in accordance with this invention, must satisfy 
two requirements. The first requirement is that the polyester not be 
depolymerized substantially; and second, that the polyester must be 
rapidly and efficiently recovered from solution by quenching. Solvents 
having 1-3 cyclic nuclei are known to dissolve polyester, including 
polyethylene terephthalate; but it was not known that such solvents would 
give up polyester rapidly and in usable form when subjected to a quenching 
action. Examples of compounds having this characteristic are diphenyl, 
diphenyl ether, naphthalene, methylnaphthalene, benzophenone, diphenyl 
methane, acenaphthene, phenanthrene, para-dichlorobenzene, and the like. 
Substituted naphthalenes and biphenyls are specifically included in this 
group. Most solvents or mixtures thereof, of this type, dissolve 
polyethylene terephthalate, for example, at temperatures of from about 
160.degree. C to 240.degree. C in an amount of about 10-40% of the polymer 
in solution. When these solutions are cooled gradually to about 
100.degree. C, a polycondensate precipitates as an amorphous gel or paste. 
On the other hand, however, when such polymer solutions are shock 
quenched, a solid white powder or flocculate is formed. 
By "non-depolymerizing dissolution solvent for polyester" is meant any 
solvent which permits of dissolution and quick precipitation of high 
molecular weight polyester (over 10,000-20,000 number average molecular 
weight) without loss of more than about 15% of such molecular weight. 
While there are other solvents which are nondepolymerizing for polyester, 
those which contain carbocyclic rings, preferably 1-3 carbocyclic rings, 
have been found particularly effective in quickly rendering a useful 
precipitate. Naphthalene has been found especially suitable for the 
practice of this invention because the solubility of polyester in 
naphthalene is a strong function of temperature, ranging from 0 solubility 
at 170.degree. C to 55 percent polyester solubility at 218.degree. C 
(boiling point of naphthalene). 
The polyester, in the form of fabric or collection of fabrics, is contacted 
with sufficient solvent under dissolution conditions for the polyester 
fibers. Apparatus or equipment which may be used for fiber dissolution 
include tanks or vats, which may be agitated or not agitated, whether open 
top or covered or sealed to hold pressure or vacuum; bowl-type washing 
machines; centrifugal filters, with provisions for solvent rinsing and 
with or without provisions for continuous or intermittent removal of 
undissolved fabric contaminants; continuously or intermittently moving 
conveyor belts passing through solvent-contacting zones; screw conveyor 
devices; and solvent spraying devices. Of course, atmospheric conditions 
are preferred. Heat and agitation will normally be required. 
When the polyester fibers have dissolved, the solution may be filtered if 
desired to remove any undissolved impurities. 
Quenching by addition is accomplished by subjecting the solution to a 
quenching medium, preferably in the form of a liquid which is preferably a 
solvent for the primary dissolution solvent. For example, a naphthalene 
solution may be shock quenched with dimethyl formamide, this method having 
the advantage that should the quenching solvent lower the temperature of 
the naphthalene to a point where it would ordinarily solidify, the 
quenching solvent would keep the naphthalene in liquid phase. Other 
suitable quenching media which are solvents for naphthalene include 
acetone, dichloromethane, 1,1,1 trichloroethane, 1,4 dichlorobenzene, 
benzene, 2-butanone, dichloromethane, dimethylacetamide, ethanol, 
methanol, tetrachloromethane, toluene, trichloromethane, xylene, and 
molten 1,4-dichlorobenzene. of course, in the case of napthalene, 
polyester will precipitate from solution at a higher temperature than that 
at which naphthalene solidifies; and therefore quenching may be done with 
a non-solvent for naphthalene. Water, for example, has been successfully 
employed as a quenching material for the polyester solution. Of course, 
the dissolution solvent and quenching medium must not react with each 
other in an explosive or otherwise hazardous manner. 
Quenching may be achieved while spinning a polymer solution through a 
quenching medium into filament with or without sequential or simultaneous 
drawing. If the quenching medium is a liquid, the solvent may be removed 
from the fiber during the fiber-forming process. If fibers are extruded 
into a gaseous quenching medium inert to the polyester as, for example, 
air, nitrogen, carbon dioxide, steam or mixtures thereof and which is at a 
temperature low enough to cause precipitation or coagulation of the 
polymer, the solvent contained in the polymer can thereafter be removed by 
extraction with a suitable solvent for the polymer but which is not, 
however, a solvent for the polyester. Suitable extraction solvents are 
inclusive of the liquid quenching media hereinbefore described. A solution 
spinning process wherein the polyester solution is spun into filaments and 
the filaments are passed through a liquid coagulating bath comprising the 
aforesaid liquid quenching media is particularly desirable. In such 
processes, the liquid quenching media may provide simultaneous 
precipitation of the polyester from solution, separation of primary 
solvent and any dyes from the polyester, and coagulation of the polyester 
solution streams into fibers. 
Referring now to the drawing, the figure is a flow sheet embracing a 
preferred embodiment of this invention. Polyester waste fabric 1 may 
include dyestuff, finish, knitting oils, etc.; and there may be 
contamination in the form of other fibers, including cotton, rayon, wool, 
nylon, acrylics, and the like. Scraps of paper or wire may also be in the 
feedstock. The waste fabric is loaded into washer-dissolver 2, which is an 
agitation-spin-dry device, preferably having a wire-mesh covered rotatable 
bowl to prevent solid matter from being pumped out with liquids, this 
machine being similar to or identical with a conventional washing machine. 
Recycled naphthalene, at a temperature of about 165.degree. C, is pumped 
from quench solvent flasher 11 into washer-dissolver 2. With agitation, 
more than half of the dye, knitting oil, finish, etc., is removed, and any 
moisture in the polyester is evaporated. The waste fabric load is spun 
out, and the spent naphthalene is recycled through flasher 3 to provide 
dye-free naphthalene for reuse in the dissolution step. Residual dyestuff, 
permanent press resins, etc., are discarded. The relatively dye-free 
naphthalene is recycled at about 190.degree.-210.degree. C into 
washer-dissolver 2 causing immediate dissolution of the polyester. The 
solution of polyester in hot naphthalene is pumped out of washer-dissolver 
2 through in-line filter 4 thereafter to be mixed in two-fluid nozzle 5 
with the quenching medium which is dimethyl formamide (DMF) and dropped as 
a precipitate into slurry tank 6. Alternately, the two-fluid nozzle 5 may 
be bypassed and the quenching medium pumped directly into slurry tank 6 
while the hot solution of polyester in naphthalene is sprayed into slurry 
tank 6. 
Washer-dissolver 2, at this point contains only solid material which was 
not carried off in solution. This may include any paper, metal scraps, 
cotton, rayon, wool, nylon or acrylic fibers which were present in the 
waste fabric. This material is removed before a new load is placed in 
washer-dissolver 2. 
The low temperature DMF injection at two-fluid nozzle 5 has caused a 
shock-quenching effect and thereby precipitated finely divided polyester 
powder which forms a suspension in the solution of DMF and naphthalene. 
Some residual dyestuff is also dissolved in the solution. 
The polyester slurry is then fed to batch centrifugal filter 7 in which, 
during the first centrifuge cycle, a polyester filter cake is formed. The 
slurry feed is then interrupted and the filter cake sprayed with 
dichloromethane to rinse off the DMF, napthalene and any residual dye. The 
white filter cake is then discharged to drying bin 8 where it is dried 
under mild heat conditions (above 40.degree. C) to produce a crumb-like 
polyester material. Intrinsic viscosity of the polyester crumb product is 
equal to the intrinsic viscosity of normal polyester spinning feedstock. 
No water or air is present in the drying bin, the vaporization of the 
dichloromethane generating its own continuous inert gas purge. Under such 
conditions, the material may then be fed directly from drying bin 8 to a 
bypass-vented extruder (not shown) and thereafter spun into filament. 
In addition to the dye purge flasher 3, recovery systems for the 
naphthalene, the quench solvent, and the rinse solvent are integral with, 
although not essential to, this invention. Included in the material fed to 
centrifuge 7 is the naphthalene polyester dissolution solvent, the DMF 
quenching medium, and the dichloromethane rinse solvent. This combination 
of solvents is fed from centrifuge 7 to rinse solvent flasher 9 wherein 
dichloromethane is vaporized at about 40.degree. C and fed to rinse 
solvent tank 10 for reuse. The DMF/naphthalene/residual dye solution is 
fed from rinse solvent flasher 9 to quench solvent flasher 11 where DMF is 
vaporized at about 153.degree. C for use in quenching the 
polyester/naphthalene solution. The residue from quench solvent flasher 11 
is fed to washer-dissolver 2 for use in the preliminary dye removal step. 
Washer-dissolver 2 and rinse solvent tank 10 are equipped with condensers 
12 and 13 respectively, thereby to recover overhead losses. 
A polyester is defined as a synthetic linear condensation-type polymer 
whose repeating units contain the ester group, 
##STR1## 
these groups being integral members of the linear polymer chain. 
Polyesters 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 glycol, cyclohexane 
dimethanol and 1,4 butanediol. Polyethylene terephthalate is preferred in 
the practice of this invention. Polyesters as used herein include 
copolymers containing repeating units of two or more different kinds such 
as copolyester-amide provided that at least two-thirds of the repeating 
units are the above-defined ester linkages 
##STR2## 
Representative examples include poly(ethylene terephthalate), 
poly(trimethylene terephthalate), poly(tetramethylene terephthalate), 
poly(ethyleneisophthalate), poly(octamethylene terephthalate) 
poly(decamethylene terephthalate), poly(pentamethylene isophthalate), 
poly(tetramethylene isophthalate), poly(hexamethylene isophthalate), 
poly(hexamethylene adipate), poly(pentamethylene adipate), 
poly(pentamethylene sebacate), poly(hexamethylene sebacate), 
poly(1,4-cyclohexylene adipate), poly(1,4-cyclohexylene sebacate), 
poly(ethylene terephthalate-co-sebacate), and 
poly(ethyleneco-tetramethylene terephthalate). 
Unless otherwise indicated, the term "polyester fiber", as used herein to 
describe the starting material which is subjected to dissolution and 
recovery in accordance with this invention, includes fiber, filaments, 
monofilaments, bands, ribbons, tubes, films and other linear constructions 
and other extruded or molded linear polyester polymer materials such as 
spinning machine lump waste, polyester flake, molded articles such as 
bottles, gears, or other solid objects (which may be processed directly or 
can be ground to pellet or powder form before being processed); 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. 
EXAMPLE 1 
To show the effect of naphthalene-type solvents on molecular weight, a 
sample of fabric composed of polyethylene terephthalate fibers was 
dissolved at 10% polymer concentration in naphthalene at 210.degree. C. 
The solution was placed under vacuum at 110.degree. C for 2 hours, and a 
fine white powder of polyethylene terephthalate was obtained. Residual 
naphthalene was rinsed off with 1,1,1-trichloroethane. The intrinsic 
viscosity of the recovered powder was compared with the intrinsic 
viscosity of the original sample, and the results are shown in the 
following table: 
TABLE I 
______________________________________ 
Intrinsic Viscosity 
Intrinsic Viscosity 
of Original Sample 
of Recovered Powder 
______________________________________ 
0.6031 0.6045 
0.6145 0.6074 
______________________________________ 
EXAMPLE 2 
A brown commercially available double knit polyester (polyethylene 
terephthalate) sample was dissolved at a 10% polymer concentration in 
naphthalene, and the solution was poured (hot) into an excess of dimethyl 
formamide at 140.degree. C, producing a dilute slurry of white polyester 
in a colored solution. The slurry was allowed to cool and was thereafter 
filtered. The solids were rinsed twice with dimethyl formamide, and then 
with water. After drying, the granular solids were an off-white color. The 
heat of the dimethyl formamide in this example apparently detracted from 
the quenching effect. 
EXAMPLE 3 
The same hot polyester naphthalene solution was poured into an excess of 
dimethyl formamide at room temperature, then filtered and rinsed with 
dimethyl formamide and water. After drying the powdery solid was white. 
EXAMPLE 4 
A 20% solution of linear polyester in phenanthrene at 210.degree. C was 
quenched in acetone, producing a white flocculent product melting at 
250.degree. C with an intrinsic viscosity of 0.5153. 
EXAMPLE 5 
A 20% solution of linear polyester in acenaphthene was quenched in acetone, 
producing a cream-colored flocculent product melting at 254.degree. C, 
with an intrinsic viscosity of 0.5418. The cream color probably was 
attributable to the brown color of the acenaphthene before heating to 
melt. 
EXAMPLE 6 
To illustrate the significance of a quench as opposed to slow precipitation 
from a product quality point of view, commercially purchased dyed 
polyester rags were dissolved at 17.25% polymer concentration in 
naphthalene. Half of this solution was poured at 200.degree. C directly 
into acetone. The other half was cooled slowly to room temperature. The 
slowly cooled material yielded a pale blue powder having an intrinsic 
viscosity of 0.52-0.54. The quenched product was a flocculent white 
product having an intrinsic viscosity of 0.52-0.53. Apparently slow 
cooling traps dyestuff within the polyester powder particles. 
As previously stated polyethylene terephthalate is the preferred polymer. 
However, copolymers are workable as are other linear and quasi-linear 
polymers such as exemplified below. 
EXAMPLE 7 
A brown commercially available double knit poly(cyclohexane dimethanol 
terephthalate) sample was dissolved at a 10% polymer concentration in 
naphthalene, and the solution was poured hot into an excess of dimethyl 
formamide at room temperature, then filtered and rinsed with dimethyl 
formamide and water. After drying, the powdery solid was white. 
EXAMPLE 8 
A 10% sample of poly(butylene terephthalate) in naphthalene at 210.degree. 
C was poured directly into acetone at room temperature. Polyester was 
recovered as a powder, having a melting point of 225.degree. C, and an 
intrinsic viscosity of 1.0660.