Separation of nylon 6 from mixtures with nylon 6,6

Mixtures of nylon 6 and nylon 6,6 are separated by solubility difference in aqueous aliphatic carboxylic acid.

BACKGROUND OF THE INVENTION 
It has been increasingly important to recycle polymeric and other consumer 
waste materials. By recovery of useful polymer from scrap material such as 
nylon carpeting, now sent to landfills, the volume of such material can be 
reduced. This lessens the burden on landfills and conserves petroleum raw 
materials. The present invention is an important step in this direction. 
SUMMARY OF THE INVENTION 
The present invention provides a method for separating nylon 6 polymer from 
a mixture thereof with nylon 6,6 polymer. The process comprises treating 
the mixture with an aqueous solution of an aliphatic carboxylic acid at a 
concentration and temperature sufficient at atmospheric pressure to 
dissolve nylon 6 polymer while leaving nylon 6,6 and separating the 
solution of nylon 6 from the nylon 6,6 polymer. As a variation of such 
process, the mixture of polymers may be treated with the acid at a 
temperature and concentration sufficient to dissolve both nylon 6 polymer 
and nylon 6,6 polymer, at atmospheric pressure, cooling the solution to 
preferentially precipitate nylon 6,6 polymer and separating the solution 
of nylon 6 from the nylon 6,6 polymer. 
DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, nylon 6 polymer is separated from 
a mixture containing both nylon 6 and nylon 6,6 polymers regardless of 
whether consumer waste products such as pigments, fillers, or carpet 
backing and the like are present. Where the polymers are in the form of 
fiber in a carpet, it is possible to treat the carpet or to shear the face 
yarn and treat only the yarn. 
Regardless of the form of the polymer mixture, it is treated with an 
aqueous solution of a 1 to 6 carbon aliphatic carboxylic acid to dissolve 
nylon 6 polymer and to leave undissolved nylon 6,6 polymer and any 
consumer waste product. Aqueous acetic acid having a concentration of from 
50 to 70% is preferred for this purpose. At such concentrations, 
extraction of the nylon 6 polymer can be achieved by reflux at atmospheric 
pressure, thereby eliminating the need for pressure equipment, while the 
dissolution of nylon 6,6 is essentially avoided. The solution of nylon 6 
is then separated from the nylon 6,6 polymer and from any insoluble waste 
products that might be present and if desired the nylon 6 recovered from 
the solution by techniques well known in the art such as dilution with 
water or reduction in temperature. A particular advantage of the process 
is that it can be conducted at temperatures well below the melting point 
of polypropylene, a material commonly employed in carpet backing. For this 
and other reasons, the acid and its concentration should be selected such 
that dissolution of the nylon 6 polymer can be achieved at reflux or lower 
temperatures. 
In an alternative process, the mixture of nylon 6 and nylon 6,6 polymer is 
treated with the carboxylic acid or an aqueous solution thereof at a 
concentration and temperature sufficient to dissolve the fiber mixture at 
atmospheric pressure. The resulting solution of nylon 6 and 6,6 polymer is 
then separated from backing or other waste, if any, and cooled to 
preferentially precipitate nylon 6,6 polymer. The precipitate is then 
separated from the nylon 6 solution by filtration, decanting or other 
means. For this embodiment the concentration of the acetic acid, for 
example, should be greater than 70%, preferably at least 80%. The 
concentration of other carboxylic acid required to dissolve both polymers 
will be readily determined by those skilled in the art. 
The process of this invention can be used with any combination of nylon 6 
and 6,6 polymer, thus readily accommodating the variability of feed which 
occurs when mixed consumer and industrial nylon waste is processed.

The invention is described further in the examples which follow. The 
examples are presented to illustrate but not to restrict the present 
invention. Parts and percentages are by weight unless otherwise noted. 
EXAMPLE 1 
This example illustrates the separation of nylon 6 from nylon 6,6 by 
selectively dissolving nylon 6 in an aqueous acetic acid solution. 
In a flask containing 270 grams of 60% acetic acid solution there was 
placed 22.5 g nylon 6 film and 15 grams nylon 6,6 fiber. This mixture was 
heated to 103.degree. C. for 15 minutes. Based on visual observation all 
of the nylon 6 film had dissolved whereas the nylon 6,6 fibers remained 
intact. The hot solution was quickly poured out of the flask, and replaced 
with 135 grams of fresh 60% acetic acid solution. The flask was then 
reheated at 103.degree. C. for 15 minutes and the hot solution added to 
the first solution. The combined decantate was allowed to cool, water 
added to enhance nylon precipitation and then filtered. The collected 
nylon powder was further washed with water and methanol and dried. The 
recovered product by differential scanning calorimetry, proved to be nylon 
6 with a melting point of 217.degree. C. No nylon 6,6 was detected. 
EXAMPLE 2 
This example illustrates the solubilization of a mixture of nylon 6 and 
nylon 6,6 in concentrated acetic acid at the solvent boiling point and the 
selective precipitation of the nylon 6,6 by cooling. 
In a flask containing 270 grams of 90% acetic acid solution, there was 
placed 15 grams of nylon 6 film and 15 grams of nylon 6,6 fiber. This 
mixture was heated to 109.degree. C. for 15 minutes at which point all of 
the nylon 6 film and the nylon 6,6 fiber dissolved. The solution was then 
allowed to cool. When the flask reached 53.degree. C., considerable solid 
material had precipitated. At this point the mixture was filtered through 
a glass fritted jacketed filter funnel which was heated by passing steam 
through the jacket. The precipitate was then rinsed with 100 grams of 
fresh 90% acetic acid solution which had been heated to 60.degree. C. 
followed by a room temperature water wash, all washes being added to the 
original filtrate. After further water washing and methyl alcohol washing, 
which was not added to the previous filtrate, the dried precipitate 
weighed 8.6 grams and based on differential scanning calorimetry, proved 
to be nylon 6,6 with a melting point of 259.degree. C. 
The collected filtrate after cooling was then filtered to recover the 
solids that had precipitated out below 53.degree. C. These solids after 
washing and drying weighed 22.6 grams and based on differential scanning 
calorimetry, proved to be a mixture of nylon 6 and nylon 6,6 with melting 
points of 218.degree. C. and 258.degree. C. respectively. The total 
enthalpy changes in joules/g indicated that it was composed of 65% nylon 6 
and 35% of nylon 6,6. 
EXAMPLE 3 
A 2 liter round bottom flask equipped with a bottom drain and heated with 
an electric heating mantle was used for this experiment. It included a 
thermocouple well in the flask to monitor temperature and a reflux 
condenser. It was situated above a 600 ml steam jacketed filter funnel on 
a vacuum filter. A mechanical stirrer was also installed in the flask 
which was initially raised up high in the flask so that it could be 
lowered and used if necessary. 
The flask was charged with 22.5 grams sheared nylon 6 carpet face fiber, 
50.3 grams whole carpet containing 22.5 grams nylon 6,6 face fiber (cut in 
pieces to fit in the flask) and a mixture of 364.5 g glacial acetic acid 
and 40.5 g water. The flask was heated electrically without stirring the 
contents. The filter funnel was heated by steam. 
On heating, the nylon 6 started to dissolve at approximately 63.degree. C. 
and on further increasing the temperature a positive indication of nylon 
6,6 dissolution was observed at 86.degree. C. On reaching 108.degree. C. 
the stirrer was lowered and rotated gently by hand to insure contact of 
all the residual backing with the solvent. The mixture was heated at 
108.degree. C. for 15 minutes and then the flask contents drained through 
the heated filter frit into the vacuum flask. On cooling to room 
temperature the filtrate solidified. The backing material remaining in the 
flask was essentially free of nylon face fiber. 
A portion of the solidified filtrate was washed two times with water and 
two times with methanol on a filter frit, to remove acetic acid, and air 
dried to give a powder. Differential scanning calorimetry of the powder 
gave total enthalpy changes in joules/g which indicated that it was 
composed of 53% nylon 6 and 47% nylon 6.6. 
EXAMPLE 4 
Example 3 is repeated except that the flask was charged with 22.5 g nylon 6 
staple fiber, 53 grams whole automotive carpet containing approximately 
22.5 grams nylon 6,6 face fiber (cut in pieces to fit in the flask) and a 
mixture of 364.5 g glacial acetic acid and 40.5 g water. On heating the 
same observations were made as Example 3. The backing material remaining 
in the flask was essentially free of nylon face fiber. 
EXAMPLE 5 
Example 3 is repeated except the flask was charged with 22.5 grams sheared 
nylon 6 carpet face fiber, 50.3 grams whole carpet containing 22.5 grams 
nylon 6,6 face fiber (cut in pieces to fit in the flask) and 810 g 88% 
formic acid. The flask was heated electrically without stirring the 
contents. The filter funnel was heated by steam. 
The nylon started to dissolve on initial contact. On reaching 87.degree. C. 
the paddle stirrer was lowered and rotated gently by hand to insure 
contact of all the residual backing with the solvent. The mixture was 
heated to 105.degree. C. over a 15 minute period and then the flask 
contents drained through the heated filter frit into the vacuum flask. On 
cooling to room temperature the filtrate remained fluid. The backing 
material remaining in the flask was essentially free of nylon face fiber.