1.8% LiCl fibres

Fibre Science and Technology 21 (1984) 295-305

Characterization of Nylon 6/1.8 ~o LiCI Fibres

F. P. La M a n t i a , D. M a n n o a n d D. A c i e r n o Istituto di Ingegneria Chimica, University of Palermo, Palermo (Italy)

SUMMARY Some fibres have been prepared in various conditions from a polyeaprolactam containing 1.8% of LiCl. Further drawing and annealing operations have been performed in order to increase their mechanical properties. A mechanical and a physico-chemical characterization has been obtained in all cases putting in evidence also the effect of moisture. The results confirm the possibility of obtaining fibres with mechanical properties larger than those of pure nylon, although not as good as those previously obtained with a larger salt content, but with a processing which is not very complicated and in some part even more favourable. Also, the behaviour of the humid fibres is not very different from that of conventional nylon 6fibres;for instance the glass transition temperature for drawn and annealed fibres is well above the ambient temperature.

INTRODUCTION In previous papers, 1-* spinning experiments have been carried out on inorganic salt/nylon 6 mixtures, obtained both from mechanically mixing a commercial polymer with the salt and from anionic polymerization of caprolactam directly in the presence of the salt. In all cases the polymeric system employed was chosen so as to give mostly amorphous fibres from highly viscous melts. 3'5'6 Further drawing and annealing of the above 295 Fibre Science and Technology 0015-0568/84/$03.00 © ElsevierApplied SciencePublishers Ltd, England, 1984. Printed in Great Britain

296

F. P. La Mantia, D. Manno, D. Acierno

fibres allowed one to obtain highly oriented fibres with a reasonable degree ofcrystallinity. 2'4 However, due to the large salt content, a drastic decrease in some physico-mechanical properties was observed when fibres were equilibrated with humid air. 7 The aim of this work was to obtain fibres with similarly high mechanical properties through a simplified processing, namely a shorter annealing, which was less influenced by moisture. The chosen system was a nylon 6 with 1-8 ')/0 LiC! which has already been characterized :4,6 it is, in fact, a system with intermediate features, especially with regard to the crystallization behaviour, between the pure polymer and the nylon with 3.7 '!,i or 4 '~/oLiC1, and is certainly a system which absorbs less humidity because of the lower salt content and the larger degree of crystallinity.

EXPERIMENTAL

Material and fibres production The material used was a polycaprolactam containing 1.8 %w/w of LiC1, obtained by anionic polymerization of caprolactam in bulk directly in the presence of the salt. s'9 A piston type of extruder, equipped with a 2.096 mm die ( L I D ~ 4) and operating under constant load, was used for the spinning experiments: the take-up apparatus was ~ l . 5 m below the spinnerette. Tests were performed at 220 °C and 260 °C with filaments extruded in air at room temperature. 2'3 For the lower temperature test, the polymer was first heated to 260°C and held at that temperature for about 10min. It was then cooled at 220 °C and finally spun: this was to destroy any possible residual crystallinity in the sample. A single flow rate was used in all the experiments ( -,~0.15 g/min) while the take-up velocity varied between 30 and 120cm/s. Further drawing of the fibres was performed in the temperature range of 70-130 °C with the aid of an lnstron machine mod. 1115 equipped with a thermostatted chamber. Drawing velocity was 10cm/min and the length between grips was initially 3 cm. Spun and drawn fibres were finally annealed at constant length at 170 °C for times of about 8 h in a vacuum oven.

Characterization of nylon 6/1"8% LiCl fibres

297

Fibre characterizations Stress-strain curves were obtained with the Instron machine operating at room temperature ( ~ 23 °C) and at an elongation rate of 0.33 m i n - 1. The measurements were carried out both on completely dry and on humid samples. The former were obtained through a permanence in a vacuum desiccator in the presence of silica-gel for at least 3 d, the latter were equilibrated with ambient humidity ( ~ 60 % R.H.) for at least 100 h. Dynamic-mechanical data were collected with a Rheovibron, mod. D D V I I C , operating at l l 0 H z and with a heating rate of about 1 °C/min. Again, measurements were performed both on dry and airequilibrated moist samples. Density measurements were performed by a density gradient column filled with solutions of n-heptane and carbon tetrachloride and operating at 22 °C.

RESULTS A N D D I S C U S S I O N Moduli of the as-spun fibres are shown in Fig. 1 as a function of the spinning draw ratio, vf/Vo, for both extrusion temperatures. In the same |

I

'

I

I

I

3

I

I

I

|

•

T = 2 2 0 "C

•

T = 2 6 0 "C

I

I

I

I

I

A A

00,

mm.

mo

•

e"

;,.. 1

I

0

Fig. 1.

I

i

I

I

!

A

•

• mm

• omo

•

I

[] • •

O lfl

I

o

•

•

oo

•

A

•

•

v f/Vo

•

,

,

I

,500

,

i

,

i

|

i

I

i

,

1000

Moduli versus spinning draw ratio. Ny 6/1.8 % LiCl at (m) 220°c and (O) 260°C, respectively. (A) Pure nylon; (A) Ny 6/3.7 ~o LiCl.

298

F. P. La Mantia, D. Manno, D. Acierno

figure, data relative to the pure nylon and the nylon with 3-7 ~o LiC1, as taken from previous w o r k s 2'4 a r e also reported for comparison. It has to be mentioned that the data given in Fig. l, as well as the birefringence, the strength and the elongation at break values discussed in the following, are average values from at least l0 measurements. Slightly larger values have been obtained with fibres spun at a lower temperature, possibly indicating a larger orientation. Also the degree ofcrystallinity is larger the lower the temperature: more particularly it resulted in about 16 ~o for fibres spun at 220 °C and with a vf/vo ~--600 and about l0 ~0 for fibres spun at 260°C and a similar spinning draw ratio. This is an interesting feature in that it indicates for the system at hand a similarity in behaviour with the pure nylon, rather than with a salted sample with higher salt concentration previously considered. 4 The nylon 6/3.7 ~0 w/w LiCl yielded fibres with a crystallinity degree slightly increasing with the extrusion temperature. However, looking again at the moduli, the values are larger than those corresponding to the pure polymer, and smaller than those obtained in presence of the 3.7 ~o LiC1. The birefringence data, given in Fig. 2, show larger values for fibres spun at 220 °C. Data relative to both unsalted and more salted samples 2'4 are also shown in this figure. They also, in view of the crystallinity degree mentioned previously for the salted samples and of the average crystallinity degree of the nylon 6, reveal a larger amorphous orientation of the fibres of the present work with respect to previous fibres. Going into more detail, they clearly confirm the larger amorphous orientation for the 2 0

i

!

i

=

s

u

~

u

[

,

,

,

,

!

u

•

•

,

~o c ,<1

10

•

"i

•

,, ::-"

•

O~ •

•

m Q

t

i

li"" ill T , Q.

0

Fig. 2.

41',lID ,

m

•

O0

IDA

vf/Vo u

,

J

I

500

i

,

i

,

I

i

1000

Birefringence versus spinning draw ratio. Symbols as in Fig. 1.

Characterization of nylon 6/1.8% LiCl fibres

299

fibres spun at lower temperature. This can be seen for instance from the formula A n = r a m ( 1 -- x ) A ° m -[-fcxA 0

A s s u m i n g f c - 0.5, for fibres at higher vf/v o ratio the crystallinity degree previously reported, A° = 74 x 10-3, as taken from the literature 1° and fam = 60 X 10 -3, average value between that of the pure nylon and that found for Ny + 4 ~ LiC1,11 we have anfa mvalue for fibres spun at 220 °C almost twice the value for fibres spun at 260 °C; faro for fibres spun at T--220 °C is about 0.19, and then practically equal to 0-18, the value obtained for nylon/3.7 ~o LiC1 spun in similar conditions. 4 Of course, the amorphous orientation for these polymeric systems is much larger than that of the pure nylon which is practically unoriented in the amorphous phase following only a spinning operation. The slightly larger value of the elastic modulus of the nylon/3.7 ~o LiCI, in spite of the lower value of the crystallinity degree and the similar values of faro andf~, can be attributed to the large amount of tie molecules present in these systems because of the more extended cross-linking. The tensile strength results shown in Fig. 3 show a negligible dependence on the extrusion temperature and a not very significant increase with the spinning draw ratio. The best observed results are only slightly larger than those for pure nylon but considerably larger than those obtained for the nylon/3.7 ~ LiCI system, x2 0.6

!

'

'

'

I

'

'

'

'

I

'

~

'

'

I

'

i

,

,

•

•

•

0.

0

0.4

= = I = = = =0 ~ 0 0 = = IOAO0 •

•O 0

i

I1%1, 0.2 ~x |

0

,

,

I

•

•

,

,

,

l

l

500

,

~

,

1000

Fig. 3. Tensile strength versus spinning draw ratio. Symbols as in Fig. 1.

300

F. P. La Mantia, D. Manno, D. Acierno

400 o ~

m

,

,

,

,

I

'

'

'

'

I

'

'

'

'

U •

AO •

,= , . A

200

&,lk)

•

A

OOo •

•

• •

k mmmmnm •

&O

-%

•

•

A

•

•

•

VdVo ,

0

,

I

,

,

,

,

I

500

,

,

,

,

I

,

,

,

,

1OOO

Fig. 4. Elongation at break versus spinning draw ratio. Symbols as in Fig. 1.

The values of the elongation at break, plotted in Fig. 4 again versus vf/Vo, indicate more clearly the differences in behaviour of salted fibres

obtained at different temperatures: more particularly fibres spun at 260 °C show values remarkably larger. In all cases, however, a ductile fracture is evidenced, similar to the pure nylon, perhaps consistent with the crystalline morphology of the system polycaprolactam 1.8 ~o LiCl. We shall recall here that, on the contrary, samples containing 3.7 ~o LiC1, like other amorphous polymers, 13 show a fragile behaviour and eventually a transition to a ductile one only at high orientation values. 4 The drawing experiments were first arranged to find optimum temperature and thus fibres obtained in spinning with a vf/v o ~- 400 were drawn at a fixed draw ratio (R = 3-5) at several temperatures. The results in terms of moduli versus temperature are shown in Fig. 5: both curves show a m a x i m u m at around 105 °C. Also, the tensile strength data present a m a x i m u m when plotted against drawing temperature (see Fig. 6). The elongation at break is not significantly dependent on temperature

Characterization of nylon 6/1"8 % LiCl fibres

301

i

5 I

O

m3

T,°¢

1

I

m

I

90

70

Fig. 5.

I

I

110

130

Moduli versus drawing temperature for fibres drawn at R= 3.5 with an elongation rate of 3.33 min- 1. Symbols as in Fig. 1.

and moreover the er values are almost the same for fibres coming out from the two extrusions (Fig. 7). The mechanical properties of the fibres can be further improved by increasing the draw ratio while retaining the drawing operation at the optimum temperature. In Table 1 average values for the modulus, the tensile strength and the elongation at break are reported together with the m a x i m u m draw ratio which could be imposed. Although the mechanical properties of these fibres are not as good as those obtained with the 3-7 ~o LiC1, they are, however, always larger than those of pure nylon. 1'2 Furthermore, although with the drawing operation the fibres reach a sufficiently large degree of crystallinity, I

I

m

o.

o

0.6

i

• •

b

Q •

I

0.4

T~ ('C

0.2

I

7O Fig. 6.

m

90

I

I

1

110

Tensile strength versus drawing temperature. Symbols as in Fig. 1.

130

302

F. P. La Mantia, D. Manno, D. Acierno I

50

30

T)=

10

i

70

90

L

i

110

i

130

Fig. 7. Elongation at break versus drawing temperature. Symbols as in Fig. 1. namely about 27 ~o and 18 ~ for the highest draw for fibres spun at 220 °C and 260 °C, respectively, the crystallinity may also be increased through a thermal treatment. 2'4 Preliminary results 14 indicate T = 170°C and t = 8 h as being the best annealing conditions. Properties of annealed fibres are shown in Table 2: an increase of the elastic modulus can be observed together with a decrease for the tensile strength and the elongation at break. In Table 3 elastic moduli are average values from at least 10 measurements. Maximum values up to 10 GPa have been obtained which are very similar to the values reached with the system Ny 6/3.7 ~o LiC1 after the same treatments. Some observations have also been made on the influence of moisture on the behaviour of such fibres. The decrease of the modulus and the increase of the elongation at break in the pure polycaprolactam, and particularly for the polymer with 4 ~o L iC17 are well-known effects. Table 3 summarizes the results of measurements performed on moist fibres. Drawn samples, both unannealed and annealed, show a drastic decrease of the elastic modulus and a smaller but still significant decrease for the tensile strength. The elongation at break is only slightly modified. It has to be mentioned that the moduli values for the humid fibres, spun at 220 °C and drawn as described above, are similar to those of the pure nylon and almost twice as large as those from Ny6/4~o LiCI. 7 Finally, some observations on the effect of drawing and annealing operations, as well as of the humidity conditions, on the glass transition temperature have been made from the dynamic-mechanical results. In

303

Characterization of nylon 6/1.8 % LiCl fibres TABLE 1 Mechanical Properties for Fibres Drawn at T = 105°C

Textr(°C)

Rmax

E ( 6Pa)

ar ( GPa)

er(~o)

220 260

4'8 5" 1

6"6 5"9

0"68 0"60

20 22

TABLE 2 Mechanical Properties of Drawn Fibres Annealed at 170 °C for 8 h

Textr(°C)

Rmax

E(GPa)

ar(GPa)

er(~o)

x(%)

220 260

4"8 5'1

8"9 7'8

0"52 0"42

17 12

44 38

TABLE 3 Effect of Moisture on Mechanical Properties of Drawn and Drawn Annealed Fibres

Textr(°C)

220 260 220"~ Annealed 260 J

E ( GPa)

a r ( GPa)

er (%)

Dry

Humid

Dry

Humid

Dry

Humid

6G 5'9 8.9 7.8

1'85 1'5 2.0 1.8

0"65 0"58 0.45 0.40

0'45 0'38 0.38 0.33

19 21 17 13

23 25 18 16

TABLE 4 Effect of Drawing and Annealing on Glass Transition Temperature (in °C) for Fibres Spun at 220°C

Dry Humid

As-spun

Annealed

Drawn

Annealed

62 30

68 38

77 42

93 61

304

F. P. La Mantia, D. Manno, D. Acierno

Table 4 data relative to fibres spun at 220 °C are reported. As before, the drawing was performed with a draw ratio R = 4.8 and the annealing temperatures and time were 170 °C and 8 h, respectively. Again a large increase with drawing and annealing is observed, and the effect is also well evident in the humid conditions.

CONCLUDING REMARKS The results of the present investigation confirm the possibility of obtaining fibres with mechanical properties larger than those of pure nylon, although not as good as those obtained with a larger salt content, with a processing procedure which is not very dissimilar from the conventional one. The extrusion temperature is certainly lower and, furthermore, when an increase in the modulus is not required the annealing procedure is unnecessary; it is, however, not a very lengthy one and may also perhaps be reduced. The decrease in the salt content is also beneficial with regard to the effect of moisture which again is similar to that observed with conventional nylon 6 fibres.

REFERENCES 1. D. Acierno, F. P. La Mantia, G. Polizzotti, G. C. Alfonso and A. Ciferri, J. Polym. Sci., Polym. Lett. Ed., 15 (1977), 323. 2. D. Acierno, F. P. La Mantia, G. Polizzotti and A. Ciferri, J. Polym. Sci., Polym. Phys. Ed., 17 (1979), 1903. 3. D. Acierno, F. P. La Mantia, G. Polizzotti, S. Russo and G. Titomanlio, J. Appl. Polym. Sci., 25 (1980), 2001. 4. D. Acierno, S. Castrovinci Zenna and F. P. La Mantia, J. Appl. Polym. Sci., 27 (1982), 1335. 5. D. Acierno, E. Bianchi, A. Ciferri, B. de Cindio, C. Migliaresi and L. Nicolais, J. Polym. Sci. Polym. Symp., 54 (1976), 259. 6. D. Acierno, R. D'Amico, F. P. La Mantia and S. Russo, Polym. Eng. Sci., 20 (1980), 783. 7. D. Acierno, F. P. La Mantia, G. Titomanlio and A. Ciferri, J. Polym. Sci., Polym. Phys. Ed., 18 (1980), 739. 8. G. Bonta, A. Ciferri and S. Russo, ACS Symp. Ser., 59, 216 Washington, D.C., 1977. 9. G. Costa, E. Pedemonte, S. Russo and E. Sav~i, Polymer, 20 (1979), 713. 10. R. Inove and S. Hoshino, J. Polym. Sci., Polym. Phys. Ed., 15 (1977), 1363.

Characterization of nylon 6/1.8% LiCl fibres

305

11. F. P. La Mantia, G. Polizzotti, G. Titomanlio and D. Acierno, J. Macromol. Sci. Phys., B21 (1982), 131. 12. S. Castrovinci Zenna, Thesis in Chemical Engineering, University of Palermo, 1979. 13. F. P. La Mantia, R. D'Amico and D. Acierno, Acta Polym., 30 (1979), 685. 14. D. Acierno and F. P. La Mantia, unpublished results.