Indian Journal of Textile ResearchVol. 8, September 1983, pp. 89-92

Extrusion of Waste Polyamides

M H SAYED-ESFAHANIDepartment of Textile Industries, Tehran Polytechnic University, Tehran

Received 14 May 1979~accepled 25 November 1982

Nylon 6 and 6.6 were re-extruded into filaments to examine the feasibility of industrial processing of waste nylon. Althoughsome degradation occurred during extrusion, satisfactory re-extrusion of both the nylons could be accomplished. Further re-extrusion caused deterioration in the characteristics of the samples, the deterioration being much more pronounced in the caseof nylon 6.6. It follows that nylon 6 (but not nylon 6.6) can be re-extruded to produce good usable textile filaments and if thediscoloration problem could be overcome, re-extrusion of nylon 6 could be a practical proposition.

Reprocessing of waste materials has become a matterof increasing economic importance to the textile andplastic industries. Though waste recovery is not a newidea, it has of necessity become more important sincethe advent of the newer synthetic fibres, primarilybecause of the high cost of the nylon ingredient. Evenchemical recovery could be thought of, if economicallyfeasible. The quantity of waste material available isestimated to be 5-10% of the total production of man-made fibres":". About 15% of the yarn used in thehosiery and knitting industry is discarded as waste,hence the importance of re-extrusion.

Experimental ProcedureNylon 6.6 chips were obtained from ICI under the

name nylon 6.6 AJ; these contained no additives.TJnpigmented nylon 6 chips were obtained fromCourtaulds Ltd.

The spinning and drawing apparatus used for meltspinning the polymer was essentially the same as thatdescribed by Copley and Chamberlain '. The chips orchopped filaments were moulded into solid rods (3/4 inin diameter, 3 in in length and 20-24 g in weight) at theoptimum moulding temperature of the polymer under1,000 lb/in? pressure to facilitate rod insertion in thespinning block.

The polymer rod was inserted into the spinningblock and extruded at the optimum extrusiontemperature and speed and then collected on thebobbin at the required speed.

Re-extrusion- The moulded polymer rods wereused as the starting material. For the first spinning, 6-8moulded rods, each 20-24 g, were prepared frompolymer chips and the first spun filament from themwas collected under the most favourable spinningconditions. The denier of the filament, the temperatureof extrusion and the other spinning variables were kept

constant; the first and the last parts of each rod werediscarded and only about three-fourths of each rodwas collected. The spun filament was chopped,moulded and used as the starting rod for the secondspinning. Similarly, the second, third and fourth re-spun filaments were used for extruding the third, fourthand fifth re-spun filaments respectively. During re-extrusion, no additives or spin finishes were used;therefore, there was no need to wash or purify thespun filament before reusing it.

Spinning conditions-For nylon 6.6, the rods, chipsand chopped filaments were sintered at 255°C andextruded at 275 ± 1°C at a constant windings rate of28 m/min through a 0.043 cm diameter jet at 9 cm/hrrod travel speed. About 120 denier filaments wereobtained.

For nylon 6 chips and chopped material, thetemperatures of moulding and extrusion were asfollows:

No. of timesextruded

I2345

Moulding temp. Extrusion temp.°C °C

210209209209209

250249248247246

All the filaments were extruded at a constant windingrate of 28 m/min, through a 0.50cm diameter jet at 8cm/hr rod travel speed and about 120 denier filamentswere obtained.

Study of .fibre properties-Draw ratio (DR) wasobtained by making successive small increases in thespeed of the second godget of the drawing frame bymeans of the kopp varia tor, until the filament finallybroke. For measuring the maximum draw ratio and

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INDIAN 1. TEXT. RES., VOL. 8, SEPTEMBER 1983

the safe draw ratio at room temperature (RT) and/or at 120°C, a short time interval (3 min and 0.5min respectively) was allowed to elapse between eachchange in speed; the filament was run continuously andthe draw ratio at which the filament broke wasmeasured.

The spun filaments were tested for regularity (CV%)using the Zellweger Uster evenness tester.

The tensile properties and elastic properties at 20%extension of drawn filaments (DR4 at 120°e) weremeasured on single fibre using an Instron tensile tester.The mean value was obtained from 10 individualtests,

The birefringence of undrawn and drawn filaments(DR 4 at l20°e) was determined by Senarmountmethod".

The melting point of polymers in the form of chipsor fibres was determined under a kaffer type hot stagemicroscope.

To determine moisture absorption, the samples wereconditioned to 65% relative humidity over a saturatedsolution of sodium nitrite at 22°C and dried overphosphorous pentoxide till equilibrium was attained.

To determine viscosity, Ubbelohde suspended leveldilution type viscometer was used. The viscosities ofsolutions of nylon 6.6 respun in 90% formic acid weremeasured at 25°C and the average molecular weightcalculated according to the equation used by Taylor5:

Mn = 13,00 ['1J 139

The viscosity of solution of respun nylon 6 wasmeasured at 25°C in 90% formic acid containing 2moles/litre potassium chloride. The number averagemolecular weight was determined using the followingequation":

['1J = K Mn0559

where K = 142 x 10 -5 dl/g.A Unicam UV visible spectrophotometer SP 500

was employed to take UV absorption spectra usingIem silica cell, and I% solution of nylon in 90'l~formic acid. The colour factor was determined usingthe equation of Mark and Clark 7:

Colour factor %

{Transmittance at _ { Transmittance at400 millimicrons 560 millimicrons

{Cell length in {concentration of

em x polymer in solution

x 100

For determining the amino end groups (in phenol/methanol 70: 30, wt/wt) and carboxyl end groups (inboiling benzyl alcohol) the potentiometric titration

90

method of Taylor and Waltz8 was used; the molecularweight was then calculated.

Results and DiscussionThe fibre properties are tabulated in Tables I and 2.

The original purpose of the present study was toexamine the possibility of recovering waste nylon bysimple re-extrusion. It has been found possible to spinfine filaments of waste nylon 6 and nylon 6.6 undernormal extrusion conditions, except in those instanceswhere the molecular weight was low (7000-8000). Thesame samples of nylon 6 and nylon 6.6 could beextruded five times and filaments obtained. However,as a result of repeated extrusion, the properties of thefilaments deteriorated progressively in the case ofnylon 6.6, but slowly for nylon 6.

The most prominent result of repeating theextrusion of both the nylons is that the materialbecomes progressively disco loured, ultimately becom-ing red-brown after five successive extrusions. This isdue to thermal oxidation. Nylon 6.6 disco lours morerapidly than nylon 6, as the former requires a highertemperature for extrusion.

For re-spun nylons, there is a good quantitativecorreIa tion, particularly in the case of nylon 6, betweenthe decrease or increase in viscosity and the formationor reduction in the number of end groups. In the caseof re-spun nylon 6.6, the viscosity, after the first andsecond extrusions, increases and the number of endgroups decreases, showing that in the manufacture ofthe chips, polymerization had not proceeded to theequilibrium stage in the rod spinner and, therefore,continues to some extent, during the manufacture ofthe rod and the extrusion process.

The viscosity average molecular weight is alwayshigher than the number average molecular weightcalculated from carboxyl end group content, since theformer approximates to the weight average molecularweight. As expected, however, re-extrusion leads tothermal oxidation and a fall in molecular weight; in thecase of nylon 6, after the first extrusion, and for nylon6.6, after the second extrusion, there is a decrease inviscosity and an increase in the number of end groups.The molecular weight falls to 13,500 (nylon 6.6) after 5extrusions, although the polymer is still capable, at thismolecular weight, of forming fibre.

Birefringence is very closely related to the natureand orientation of the molecules. The birefringence ofundrawn re-extruded nylons falls and the fall in thecase of nylon 6.6 is higher than that for nylon 6, whichis probably due to the more rapid degradation of nylon6.6. But after drawing (DR 4 at l20°e), the value ofbirefringence increases, the increase being higher in thecase of nylon 6.6 than nylon 6. This is probably due to

SAYED-ESFAHANI: EXTRUSION OF WASTE POLYAMIDES

Table I-Fibre Properties

No. of

Draw ratio (DR)IrregularityTensile propertiesInitialElastic propertiestimes

Young'sextru-

Max. DR Max. DR Safe DR UndrawnDrawnTena-Exten-Work of modulusElasticElasticWorkded

at 120°Cat R.T.at 120°Cfila-fila-citysibilityruptureg/dexten-reeo-reeo-ment

mentg/d%g/d sionveryveryCV%

CV% %%%

Nylon 6.61

5.43.44.8~.46.74.9861.62.3731.013.567.524.12

5.13.04.45.710.04.3651.31.8828.213.065.023.33

4.41.83.86.911.94.2745.31.5234.012.060.031.04

4.21.43.510.613.04.1336.61.2042.011.758.519.85

3.31.12.811.617.23.7227.20.8341.411.557.518.4

Nylon 61

5.55.05.29.711.85.8049.62.1337.015.075.042.32

5.65.05.010.712.45.3551.12.1037.415.577.539.43

5.14.84.811.512.35.3350.41.9737.015.376.540.14

5.14.84.812.212.25.2045.91.7827.815.577.539.45

4.64.24.416.320.14.9445.51.6626.615.477.044.1

Table 2-Fibre PropertiesNo. of

Birefringence (10")Moisture regainMeltingColourViscosity End groupstimes

pointfactorextru-

UndrawnDrawnUndrawnDrawn°C%LimitingMnAminoCarboxylMnded

filamentfilamentfilamentfilament visco-endend%

%%% sitygroupgroup(,,)

meq/kgmeq/kg ,

Nylon 6.6Chips

--0.01.22017,10045.179.112,6501

385424.303.97262.00.01.24017,55043.873.613,6002

185754.283.97262.02.31.26017,94039.169.914,6003

156074.243.94261.04.51.18016,36042.968.114,7004

176164.253.91261.07.31.11015,00043.868.114,7005

116024.243.91260.59.81.02513,48045.775.513,500

Nylon 6

Chips

-- -0.00.34017,80047.569.014,5001

245115.104.49224.00.70.37021,00036.858.917,0002

105115.184.52224.01.50.36020,00037.362.616,0003

115215.214.46224.02.00.33017,00040.564.415,0004

105335.064.50224.04.00.33216,02044.769.914,3005

85365.034.58224.07.30.32016,00042.970.814,100

the influence of successive extrusions on the

drawability of the material.No significant changes occurred in the moisture

uptake and melting points of re-spun nylons. The fallin moisture regain by drawn filaments (in comparisonwith undrawn filaments) shows that drawing improvesthe orientation of the crystallites and thus reduces theaccessibility of moisture.

The CV values of the first extruded nylon 6.6 andnylon 6 filaments, before drawing, were 6.4% and 9.7%and after drawing (DR 4 at 120°C) they were 6.7% and11.8% respectively. These figures gradually increase onsubsequent re-extrusion. The higher CV values of

nylon 6 are related to the different spinning conditionsused for the two polymers3.9. Bobeth and Clement 10

found that the irregularity of nylon 6 is unaffected bydrawing; this was also found to be true for nylon 6.6filaments resulting from the first extrusion-6.4""before drawing and 6.7~,~after drawing. However. insubsequent extrusions, the difference between theirregularities of the drawn and undrawn nylon 6.6filaments increased steadily. Since the only changesoccurring in the samples are those due to thermaldegradation, it seems likely, as suggested by Freemanand Coplan II, that some gel material is being formedwhich leads to inhomogeneity in the melt. This is found

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INDIAN J. TEXT. RES., VOL. 8, SEPTEMBER 1983

to be true, as difference between the irregularities ofdrawn and undrawn nylon 6 becomes detectable afteronly five extrusions.

It is, however, to be expected that an increase in yarnirregularity would influence the drawing ability, i.e. themaximum draw ratio attainable. Themfore, possiblysome relationship exists between the maximumattainable draw ratio and the filament irregularity.

The changes in mechanical properties on re­spinning nylon 6.6 are: (i) reduced tenacity,extensibility, work of rupture, elastic and workrecovery from 20% extension, and (ii) increased initialYoung's modulus values.

The changes in mechanical properties on re­spinning nylon 6 are: (i) slightly reduced tenacity, workof rupture and extensibility (in the 4th and 5th re-spunfilaments only) and a reduced initial Young's modulus(also in the 4th and 5th re-spun filaments), and (ii)elastic and work recoveries from 20% extension offilament do not show any significant changes.

Thus? the stress-strain characteristics of re-spunnylon 6'varied little from one re-extrusion to another,but the variations in the re-spun nylon 6.6 aresignificant. It may be concluded that the fall in tensilestrength and elastic behaviour as well as the brittlenessof the re-spun nylon 6.6 are to some extent due to thefollowi~g: (i) increase in irregularity due to the non­homogeneity in the melt stream, (ii) loss of lowmolecular weight polymers or monomers which causesbrittleness and reduces extensibility and (iii) thermaldegradation during re-extrusion, lowering themolecular weight of the polymers.

92

Conclusion

Re-extrusion of waste nylon 6.6, particularly ofnylon 6, appears to be a feasible operation. Minorchanges occur in the chemical characteristics as a resultof five extrusions. The main problem is discolorationand this can also be reduced partly by usingcommercial melt spinning equipment and partly byadding a suitable antioxidant to the polymer meltbefore extrusion or by using a suitable bleaching agentfor the extruded filaments.

In nylon 6, apart from discoloration, there is noproblem in re-extrusion. The present study, however,concerns simple re-extrusion in which the filaments aretotally uncontaminated by dyes and finishing agents.In industrial practice, the waste would becontaminated with either of these or both.

References

I Annual Report, Wool & Wool/ens of India, 10 (1968) 25.

2 Diba V & Vavacek M, Man-Made Textiles, 45 (1961) 443.

3 Copley M & Chamberlain N H, Appl Polymer Symp. 6 (1967)27.

4 Hallimond A F, in Manual of thepolansmg microscope, 2nd Ed.,edited by G. Cook (Troughton & Simms Ltd), 1950.

5 Taylor G, JAm chem Soc, 69 (1947) 635.

6 Saunders P R, J Polym Sci, A2 (1964) 3755.

7 E I Du Pont De Nemours & Co, Br Pat 708.029, 28 April 1954.

8 Taylor G B & Waltz J E, Analyl Chem, 19 (1947) 448.

9 Ziabicki A & Kedricerska K, J apal Polym Sci, 6 (1962) Ill.

10 Bobeth W & Clement A, Faserlarschung und Texri/lechmed. 7(1968) 293.

II Freeman H & Coplan M J, J appl Polym Sci. 8 (1964) 2389.