Recovery of polyamides from composite articles

A process for recovering polyamide from composite articles that contain polyamide involves (a) subjecting at substantially nondepolymerizing conditions the composite article to a solvent for polyamide for a time sufficient to dissolve substantially all of the polyamide and leaving an insoluble fraction; (b) separating insoluble fraction of the composite article from the dissolved polyamide; and (c) precipitating the dissolved polyamide with appropriate precipitants.

RELATED APPLICATION 
The present application claims the benefit of U.S. provisional application 
Ser. No. 60/000,667, filed Jun. 29, 1995. 
FIELD OF THE INVENTION 
The present invention relates generally to recovering polyamide from 
polyamide-containing composite articles. 
BACKGROUND OF THE INVENTION 
As used herein, the term "high purity" means a purity level greater than 
90% as measured by the Kjeldahl nitrogen analysis method. 
As used herein, the term "nondepolymerizing conditions" means conditions 
that do not result in a relative viscosity (RV) decrease of greater than 
25% as measured in 90% formic acid in a modified ASTM 0789 test (1% 
polymer solution). 
Polyamides are used in many articles. Nylon 6 and nylon 6,6 are 
representative polyamides and are among the most widely used. These 
polyamides are present in, among other things, a wide range of fabrics, 
carpets, films, moldings, etc. Most often, however, polyamides are not the 
exclusive material present. They are commonly combined with other 
plastics, fillers, additives, etc. For example, carpets made from nylon 
face fiber often include polypropylene, latex and calcium carbonate as 
backing materials. 
The recovery of polyamides is gaining in importance as environmental 
awareness increases and landfills become full. It is preferred to recover 
polyamides in as high a purity as possible. The higher the purity, the 
broader the opportunity for reuse. Recovery of pure polyamides from 
composites, however, has remained difficult. Because full separation of 
polyamide remains difficult, many methods of recovering polyamides deal 
with leaving it in composites that are used for low quality products like 
pallets and building materials for gardens and landscaping. It is 
desirable, as noted already, to be able to use recovered polyamide to make 
materials that require high grade nylons, like fibers. 
The recovery of fiber grade caprolactam from nylon 6 face yarns is 
described in U.S. Pat. No. 5,169,870 to Corbin et al. The recovery of 
caprolactam from mixed materials using dissolution is described in U.S. 
Pat. No. 5,241,066 to Davis et al. 
There remains a need for ways to recover polyamides in a form pure enough 
to be reused without limitation on the products made from the recovered 
polyamide, e.g., fibers or thin-walled injection moldings. 
SUMMARY OF THE INVENTION 
The present invention addresses the needs in a process for recovering 
polyamide from composite articles that contain polyamide. The method 
involves (a) subjecting, at substantially nondepolymerizing conditions, 
the composite article to a solvent for polyamide for a time sufficient to 
dissolve substantially all of the polyamide and leave an insoluble 
fraction of the composite article; (b) separating insoluble fraction of 
the composite article from the dissolved polyamide; and (c) precipitating 
the dissolved polyamide with appropriate precipitants. 
It is an object of the present invention to provide a process for 
recovering high purity polyamide from polyamide-containing products and 
composites.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To promote an understanding of the principles of the present invention, 
descriptions of specific embodiments of the invention follow and specific 
language describes the same. It will nevertheless be understood that no 
limitation of the scope of the invention is thereby intended, and that 
such alterations and further modifications, and such further applications 
of the principles of the invention as discussed are contemplated as would 
normally occur to one ordinarily skilled in the art to which the invention 
pertains. 
In the present invention, a composite product containing polyamide is 
dissolved in a solvent which is generally not a solvent for the 
non-polyamide components of the composite or which does not generally 
dissolve the non-nylon components of the composite at the conditions 
(e.g., temperature, dissolution time and solvent concentration) used. The 
dissolved polyamide is separated from the still solid non-polyamide 
components of the composite. The polyamide is recovered from solution by 
precipitation. 
Useful solvents for the nylon component include known polyamide solvents 
such as inorganic acids, organic acids, phenols, cresols, alcoholic 
solutions of alkaline earth halides, glycols (e.g., ethylene glycol), or 
aromatic alcohols. Useful organic and inorganic acids include, but are not 
limited to, formic acid (60-100%), sulfuric acid (30-60%), phosphoric acid 
(70-85 wt. %), hydrochloric acid (15-40%), acetic acid, nitric acid, etc. 
Useful aromatic alcohols include phenylmethyl alcohol, phenylethyl alcohol 
and benzyl alcohol. Other solvents include lactams (e.g., caprolactam) or 
lactones. Combinations of these solvents may also be used. The currently 
preferred solvent is hydrochloric acid (15-40%). 
When the solvent is a strong acid, elevated temperatures and long 
dissolution times should be avoided, since degradation of the polyamide 
may take place and make recovery from solution difficult. If heating is 
used at all, it should be moderate (about 20.degree. C. to about 
100.degree. C.). Temperatures between 20.degree. C. and 60.degree. C. are 
preferred. 
The length of time for the dissolution varies. For practical reasons and to 
minimize degradation, the short times are preferred. The time required for 
substantially complete dissolution will, of course, depend on the 
conditions (size of sample, acid type, concentration, temperature) used. 
Generally, dissolution times on the order of 2-30 minutes are preferred. 
The polyamide in the composite is dissolved in an appropriate solvent. The 
undissolved fractions are then separated from the dissolved polyamide 
solution. Separating the fractions may be accomplished through filtration 
or gravity settling. It is contemplated that the undissolved fraction can 
be separately reprocessed, either by further separation of the components 
or by compounding the material as is. 
The dissolved polyamide fraction obtained from the separation step is 
treated with appropriate precipitating conditions to yield a polyamide 
precipitate. The precipitating conditions may include conventional polymer 
precipitating agents such as dilute acids, water, short chain aliphatic 
alcohols, or combinations of these. The precipitating conditions may also 
involve cooling. For example, if hot solvents are used in dissolution, it 
is advantageous to achieve precipitation by cooling. 
The precipitate may be subjected to gentle drying. Especially beneficial is 
drying in a vacuum or in the presence of an inert gas at reduced 
temperature. Drying in vacuum or in the presence of an inert gas at a 
temperature ranging between 40.degree. C. and 100.degree. C. was found to 
be of benefit. If acid is used as the solvent, neutralizing residual acid 
and washing the filter cake with water may be beneficial. 
Further processing of the solvent recovered after filtration of the 
polyamide precipitate is also possible. Distillation with separation of 
the solvent from the precipitating agent may be necessary, but this 
procedure makes possible full recycling of all materials used in the 
process. The precipitating agent may be recycled to be used in a new 
precipitation phase, and the solvent may be recovered for renewed use. 
It may, in some circumstances, be advantageous to first reduce the size of 
the composite scrap. A preliminary mechanical separation step is 
advantageous for such size reduction. Such a step may be accomplished 
using a guillotine cutter, rotary cutter, shredder, grinder, hammermill 
and vibrating screens or a series of combinations of these. 
FIG. 1 illustrates schematically an exemplary process of the present 
invention practiced on a carpet. As shown in FIG. 1, a carpet containing 
nylon 6 may be first reduced in size by, for example, shredding 10. 
Following such optional size reduction, the carpet is subjected to 
dissolution 12, where the nylon 6 is dissolved from the remainder of the 
carpet. Dissolution using 20% HCl is shown. The dissolved nylon 6 is 
separated from the insoluble materials (like the carpet backing) by, for 
example, filtration 14. Precipitation 16 follows next to recover the nylon 
6 from solution. In filtration 18, nylon 6 is separated from the 
supernatant. Neutralization, washing/drying step 20 follows to yield 
relatively pure recovered nylon 6. 
Additionally, as also shown in FIG. 1, the solvent (here HCl) may be reused 
in the process. If dilution of the solvent with water is used to cause 
precipitation in step 16, distillation 22 may be used to recover water for 
reuse in precipitation step 16 and the reconcentrated acid routed back to 
dissolution step 12. As indicated, if desired, the calcium chloride which 
also is dissolved in the solvent may be recovered. 
The invention will be described by reference to the following detailed 
examples. The Examples are set forth by way of illustration, and are not 
intended to limit the scope of the invention. In the examples, all parts 
are part by weight unless otherwise specified. As demonstrated in the 
following examples, the process of the invention leads to the recovery of 
polyamide exhibiting properties comparable to those of virgin polyamide. 
The following methods are used in the Examples: 
Relative Viscosity: Relative Viscosity (RV) is measured in 90% formic acid 
using a modified ASTM 0789 test (1% polymer solution). 
Nylon Purity: Nylon Purity is measured by the Kjeldahl nitrogen analysis 
method. 
EXAMPLES 1-4 
RECOVERY OF NYLON 6 USING HCL 
A piece of nylon 6 face yarn carpet is placed in 20% hydrochloric acid at 
40.degree. C. and dissolution times of 2 to 30 minutes. The nylon face 
fiber dissolves very readily and quickly, leaving the primary backing, the 
latex with the calcium carbonate filler and the secondary backing intact. 
Some of the calcium carbonate reacts with the hydrochloric acid to form 
calcium chloride. The solution is filtered to remove insoluble materials. 
The hydrochloric acid solution, with the dissolved nylon, is diluted with 
water. When the dilution reaches 12 to 13%, the nylon begins to 
precipitate. Initially, the precipitate is in the form of viscous, pitch 
like, sticky fluid. Upon further dilution, the precipitate begins to 
solidify in the form of film and particulate matter. Dilution to about 5% 
results in near complete precipitation. The diluted hydrochloric acid 
solution is concentrated by evaporation for reuse. 
The precipitated nylon is subsequently filtered, neutralized, washed with 
water several times and dried. The intact carpet backing is also washed 
and dried. The purity of the recovered nylon and the relative viscosity 
are measured. The results are presented in Table 1. 
TABLE 1 
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Dissolving *Nylon RV 
Example Time, min. Purity, % 
(in formic acid) 
______________________________________ 
1 2 93.1 2.48 
2 5 94.3 2.73 
3 15 91.5 2.54 
4 30 90.8 2.54 
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*By this method, pure nylon 6 chips typically result in about 95% purity. 
EXAMPLES 5-9 
RECOVERY OF NYLON 6 AND NYLON 6,6 USING H.sub.2 SO.sub.4 
For examples 5-7, nylon 6 face yarn carpet is placed in sulfuric acid at 
concentrations of 40% to 60%, temperatures of 80.degree. C. to 104.degree. 
C., and dissolution times of 5-10 minutes. For examples 8 and 9, nylon 6,6 
face yarn carpet is placed in sulfuric acid at concentrations of 50 and 
60% at temperatures of 89.degree.-100.degree. C. for 5 minutes. The nylon 
face fiber dissolves very readily and rapidly, leaving the primary 
backing, the latex with part of the calcium carbonate and the secondary 
backing intact. Part of the calcium carbonate reacts with the sulfuric 
acid to form calcium sulfate. 
The sulfuric acid solution, with the dissolved nylon, is diluted with 
water. When the dilution reaches 22 to 27%, the nylon, in both cases, 
begins to precipitate. 
In the case of nylon 6, the precipitate is in the form of viscous, sticky, 
heavy fluid which settles at the bottom of the vessel. The diluted 
sulfuric acid in this case is decanted and as soon as the precipitate is 
exposed to water, the precipitate solidifies in the form of a film which 
is easily removed washed and dried. 
In the case of nylon 6,6, the precipitate is in the form of small particle 
suspension in the diluted acid which is readily recovered by filtration. 
The RV and purity are measured as described in Example 1. The results are 
reported in Table 2: 
TABLE 2 
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Acid Acid Nylon Dissolution 
Example 
Conc., % Temp., .degree.C. 
RV Purity, % 
Time, min. 
______________________________________ 
NYLON 6 
5 60 104 2.71 92.0 5 
6 50 102 2.75 89.8 5 
7 40 80 2.65 85.6 10 
NYLON 6.6 
8 60 100 2.07 100 5 
9 50 89 2.38 98 5 
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EXAMPLE 10 
QUANTITATIVE RECOVERY OF NYLON 6 WITH HCL 
125 grams of post consumer nylon 6 face yarn commercial carpet is placed in 
700 ml of 25% hydrochloric acid at 50.degree. C. and stirred for 20 
minutes. The pieces of backing are removed, washed with water to remove 
the acid and dried. The acid solution with the dissolved nylon is filtered 
to remove residual carpet backing. The filtrate is diluted with water to 
about 8% to precipitate the nylon. The diluted acid is decanted and 
filtered. The precipitated nylon is neutralized with a dilute solution of 
sodium hydroxide, washed with water and dried in an oven at 80.degree. C. 
for 3-4 hours. 
The recovered nylon has 93.9% purity and 2.73 RV. 92.6% of the nylon face 
fiber from the carpet is recovered. 
EXAMPLE 11 
QUANTITATIVE RECOVERY OF NYLON 6 WITH FORMIC ACID 
100 grams of post consumer nylon 6 face yarn commercial carpet is placed in 
1250 ml of 85% formic acid at 40.degree. C. and stirred for 30 minutes. 
The pieces of backing are removed, washed with water to remove the acid 
and dried. The acid solution with the dissolved nylon is filtered to 
remove residual carpet backing. The filtrate is diluted with 27% formic 
acid to precipitate the nylon. The diluted acid is decanted and filtered. 
The precipitated nylon is neutralized with a dilute solution of sodium 
hydroxide, washed with water and dried in an oven at 80.degree. for 3-4 
hours. 
The recovered nylon has 91.5% purity and 3.00 RV. 92.2% of the nylon face 
fiber from the carpet is recovered. 
EXAMPLES 12-15 
For example 12, 30 grams of nylon 6 face yarn commercial carpet having a 
nylon content of 44% as measured by the Kjeldahl method and a remainder of 
polypropylene primary and secondary backing, latex binder and calcium 
carbonate filler was placed in 200 grams of 20% hydrochloric acid at a 
temperature of 20.degree. C. The carpet pieces were kept in the acid for 
20 minutes with mechanical stirring. The carpet backing was removed, 
washed to remove the acid and discarded. The acid solution with the 
dissolved nylon was filtered and diluted with 225 ml of water to 
precipitate the nylon. The precipitated nylon was recovered by filtration 
which was subsequently neutralized, washed and dried. 
For example 13, the procedure of example 12 was followed, except the 
dissolving time was 10 minutes and the temperature of the acid was 
50.degree. C. 
For example 14, the procedure of example 12 was followed, except the acid 
concentration was 30% and the temperature of the acid was 80.degree. C. 
The acid was diluted with 375 ml of water. 
For example 15, the procedure of example 12 was followed, except the acid 
concentration was 25% and the temperature of the acid was 80.degree. C. 
Dissolving time was 30 minutes. The acid was diluted with 290 ml of water. 
The RV, purity of the recovered nylon, and amount of recovered nylon are 
reported in Table 3. 
TABLE 3 
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EX- TIME CONC. TEMP. RECOVERY 
PURITY 
AMPLE (MIN.) (%) (.degree.C.) 
(%) (%) RV 
______________________________________ 
12 20 20 20 70.8 92.9 2.80 
10 20 25 50 92.6 93.9 2.73 
13 10 20 50 73.8 87.1 2.59 
14 20 30 80 97.7 88.6 2.10 
15 30 25 80 97.9 88.7 1.88 
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EXAMPLE 16 
COMATIVE 
13.5 grams of nylon 6 chip (RV=2.7) is placed in a mixture of 100 grams of 
85% phosphoric acid and 21.4 grams of water and heated to 121.degree. C. 
in 17 minutes, while stirring. Since only partial dissolution is observed, 
an additional 60.5 grams of 85% phosphoric acid is added and the mixture 
is heated to 128.degree. C. Complete dissolution of the nylon 6 chip is 
obtained after an additional 19 minutes of stirring. 
Heat is then removed and the mixture is allowed to cool to 40.degree. C. 
over a period of 59 minutes. No precipitate is observed. Water is added to 
the cooled mixture; however, no precipitate is observed. 
No precipitate can be produced until large volumes of water are added to 
the mixture, indicating that significant polymer degradation has occurred.