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Thermoplastic polyetherimide – a helpful aid for the drawing of fluoride fibres
Journal of Non-Crystalline Solids 284 (2001) 139±145
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Thermoplastic polyetherimide ± a helpful aid for the drawing of ¯uoride ®bres J. Kraus *, C. Bruschi, E. Billi Agilent Technologies, Via G. Reiss Romoli 274, I-10148 Torino, Italy
Abstract A coating layer of thermoplastic polyetherimide (PEI) of less than 40 lm prevents ¯uoride preforms from moisture attack during the drawing process. If over-coated in-line with acrylate, the bending strength of the ®bres, which was seen to depend strongly on the drawing speed, reached 1 GPa even under nitrogen atmosphere. A correlation between ®bre strength and layer thickness was found. Furthermore, a strength comparison with ¯uoride ®bres coated with a ¯uorinated copolymer was made. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.81.Bm; 61.41.+e; 62.20.Mk
1. Introduction One of the problems in the drawing of ¯uoride ®bres is the formation of a corrugated skin on the surface of the preform neck-down in the hot zone of the drawing furnace (see micrographs in [1±3]). This skin, which was reported to be amorphous [2] as well as crystalline [4], may form as early as the preform head necks down to a gob to initiate ®bre drawing or during ®bre drawing. Generally, as drawing proceeds, it is torn o by the pulling force into small ¯oe-like islands scattered over the ®bre, which constitute one of the main ¯aw sources and aect the mechanical strength of the ®bre [1,5]. Moisture in the drawing atmosphere reacting with the preform and inducing hydrolysis was con®rmed to be the reason for its formation [3±5].
* Corresponding author. Tel.: +39-011 229 2447; fax: +39-011 229 2434. E-mail address: [email protected] (J. Kraus).
Hence, work on the fabrication of high-strength ¯uoride ®bres has been directed to keeping away or removing moisture from the preform surface. Control of the drawing atmosphere, special preform surface treatments and development of protective glass over-claddings or polymeric coatings were the main approaches undertaken ([3,4,6±8] and references therein). In this paper, we present our results on avoidance of skin formation by coating the preforms with thin layers of a thermoplastic polyetherimide (PEI) before drawing. This polymer, which is completely amorphous (Tg 215°C) and soluble [9], is stable to temperatures 490°C [10], hence greater than those normally employed for ¯uoride ®bre drawing [11]. The mechanical properties of the resulting ®bres, which for the most part were again over-coated in-line with an ultraviolet (UV)-curable resin, are compared to those of bare ®bres and ®bres coated with ¯uorinated ethylene±propylene copolymer (Te¯on FEP).
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 3 9 3 - 3
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J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
2. Experimental The nine ®bres examined were drawn from multimode preforms or clad rods of the ZBLYAN type. Multimode preforms consisted of Er3 doped core glasses of the ZBLYALi composition and of clad glasses of the ZBLYAN type [12,13]. All the preform surfaces were treated in the same manner by both mechanical polishing and chemical etching [13] and then covered by Te¯on FEP (P®ster) or PEI (Ultemâ 1000, General Electric Plastics Italia), respectively. In the former case, the glass preform was ®tted into a FEP tube, which was then ®tted into another FEP tube. In the latter, the preform was dipped at room temperature into a solution of 1 g of PEI in 50 ml of N ; N -dimethylacetamide, which was prepared previously by dissolving the dried polymer with stirring at 60°C and cooling to room temperature. After removing the preform from the solution, the solvent was evaporated under vacuum. This dip-coating step was repeated 5 or 10 times. The coated preform was mounted on the drawing tower, ¯ushed with nitrogen, which was used without further drying, for 2 h and then drawn into ®bre under nitrogen atmosphere. As for the heating system, a radiofrequency (RF) induction furnace was used. Only ®bres from dip-coated preforms were partially over-coated in-line with a UV-curable urethane acrylate resin (DSM Desotech, Desolite 3471-314). Table 1 summarises the preforms used and the drawing conditions employed.
Fibres from preforms 5 and 6 were stored in clean room conditions at 21°C and a relative humidity of 50%, all the other ®bres in laboratory atmosphere. Mechanical properties were determined from two-point bending tests (Lloyd Instruments LR5K) on samples of 0.2 m length by calculating the fracture stress by [14] r 1:198 E
d
df dc 2dg
1
with r, E, df, d, dc, 2dg being the fracture stress (GPa), the Young's modulus (50:4 0:2 GPa [4]), the glass ®bre diameter (lm), the distance between platen at ®bre fracture (lm), the overall ®bre diameter including any coating (lm), and the total depth of the two platen grooves (800 lm), respectively. If not stated otherwise, the platen velocity was 10 mm/min. Generally, the fracture stress was determined on consecutive ®bre lengths of 4±5 m, i.e., 20±25 samples, and the data treated by Weibull statistics. Furthermore, fracture stress data were corroborated by analysis of the fracture surface of the broken ®bres. As known [15], the distance, r, from the fracture origin, which is in the centre of a smooth region with circular or half-circular shape called the mirror, to the mist-hackle region outside the mirror, is related to the fracture stress by the empirical equation [15±17], p r M= r;
2
Table 1 Coated preforms and drawing conditions used
a
Preform no.
Preform con®guration
Preform coating (thickness (lm))
Drawing conditions: gas ¯ow (l/min) / neck-down temp. (°C) /drawing speed (m/min) /drawing tension (g)
1 2 3 4 5 6
Cladding rod Cladding rod Multimode, rod-in-tube Multimode, rod-in-tube Multimode, rotational casting Multimode, rod-in-tube
None FEP (1150) FEP (1150) PEIb (n.m.a ) PEI (24 6, 4:0 0:3)c PEI (41 3, 10)d
1.5 / 315 / 0.4 / 0.4 7.5 / 335 / 4.0 / 0.5 7.5 / 334 4 / 7.4 / n.m.a 7.5 / n.m.a / 16.3 / n.m.a 7:5=354 1=5:2 0:7=28 3 7:5=329 4=5:3 0:3=23 2
Not measured. Five layers. c Five layers; thickness measured with a pro®lometer (Alpha step 200 (Tecnor Instruments)) on both ends of the preform (®rst value refers to the preform head). d Ten layers over half the preform; thickness measured with a mechanical gauge (Tesamaster) on ®lms peeled o from both ends of the preform (®rst value refers to the preform head). b
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
141
whose mirror radii were measured by scanning electron microscopy (SEM). The slope gives M 1:215 MPa m1=2 , which is in agreement with M 1:24 0:04 MPa m1=2 as found in [16]. Since the measurement of mirror radii is much more convenient by optical microscopy rather than SEM but less precise, a second calibration curve was established (Fig. 2) to correlate the radii revealed by optical microscopy to those measured by SEM. The curve was valid for radii up to 20 lm. Larger radii were used as measured. 3. Results
Fig. 1. Mirror size vs fracture stress (calibration curve 1). Uncertainties are less than the size of the data points. The curve is a least squares ®t of the function, mirror size A fracture stress, to the data. The correlation coecient of the ®t is R 0:978.
where M, the mirror constant, depends on the material [15]. p Fig. 1 shows the calibration curve in which 1= r is plotted against r for 17 ®bres,
Fig. 2. Mirror radius by SEM vs mirror radius by optical microscope (calibration curve 2). The data are ®tted best by a second-order polynomial (y 0:538x 0:024x2 ). The correlation coecient of the ®t is R 0:990.
In view of the large dierence in the coating thickness of FEP-coated preforms to PEI-coated preforms (Table 1), we decided to keep the gas ¯ow constant rather than the neck-down temperature during drawing. Temperature pro®le measurements made by us previously on both FEP-coated and bare multimode preforms in the proximity of the neck-down zone had shown that, only in case of FEP, a radial temperature dierence of up to 20° between the clad surface and the core/clad interface can occur. The gas ¯ow rate was set for stable drawing with no undercooling of the neck-down zone. Acceptable ®bre-diameter ¯uctuations could be expected. This process worked well for all the preforms except for bare preform 1, whose ¯ow rate had to be reduced to 1/5 of the set ¯ow rate. Preform 6 was only partly covered with PEI to obtain a maximum of ®bre variety, i.e. ®bres with PEI, ®bres with PEI and acrylate, and ®bres without PEI but with acrylate, from the same preform. Table 2 gives an overview of the ®bres obtained in terms of ®bre geometry, initial fracture strength and Weibull distribution parameters (m's), and Fig. 3(a) and (b) present their Weibull plots. Fig. 4 shows the corresponding plots of fracture strength vs mirror size for some of the ®bres. As expected, bare ®bre 1 had the smallest strength and the largest diameter ¯uctuations of all the ®bres fabricated. Continuous ®bre drawing was quite dicult, since the ®bre broke repeatedly during its winding on the take-up drum. By contrast, PEI-coated ®bres 5P and 6P, whose
142
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
Table 2 Geometry, initial fracture strength and Weibull parameters of the ¯uoride ®bres drawn Fibre noa
Fibre length investigated (m)
Mean glass ®bre diameterb (lm)
Mean overall ®bre diameterb (lm)
Fracture strength, r0 c (GPa) (samples)
Weibull parameter, m
1 2F 3F 4PU
3 5 15 22d
151 25 121 4 125 4 149 8
151 25 149 5 154 6 200 5
5P 5PU 6P 6PU 6U
10 19g 7 28h 14
143 20 177 9 130 15 135 7 143 11
143 20 227 5 130 15 207 5 216 7
0:26 0:01 0:47 0:01 0:45 0:01 0:62 0:01 1:00 0:03 0:50 0:01 0:47 0:01 0:58 0:01 0:45 0:01 0:37 0:01
8:0 0:6 5:2 0:4 3:3 0:3 11:5 0:7 6:3 0:2 2:9 0:2 6:7 0:4 2:6 0:2 15:7 0:7 12:1 0:6
(14) (24) (24) (24)e (19)f (20) (23) (20) (25) (25)
a Preform numbers and ®bre numbers are corresponding. Since from preforms 5 and 6 several ®bres with dierent coatings have been obtained, the letters F, P and U after the numerals indicate inner and outer coating and stand for Te¯on FEP, PEI, and urethane acrylate, respectively. b Data from all the bending tests; the ®bre length is given in column 2. c r0 corresponds to a failure probability of 63.2%. If not stated otherwise, the bending tests were carried out the day after drawing. d Total ®bre length 50 m. e Fibre ®rst measured after 30 days. f Measured after 97 days. g The ®bre was investigated from the end of drawing versus the beginning. h The ®bre was investigated from the beginning of drawing versus the end (total ®bre length 100 m).
coating thickness was less than 1 lm, could be pulled continuously without problems, had less diameter variations than ®bre 1 and had the largest strength range extending to 800 MPa (Fig. 3(b)). Consequently, their ms were smallest. If these ®bres were coated in-line with acrylate, the ms would increase up to ®ve times as much (®bre 6PU) and the mean bending strength was 450 MPa. The m remained large even after the transition from dual PEI/acrylate-coating to single acrylate coating (®bre 6U), whilst the strength decreased by 80 MPa. Fig. 3(a) shows that both PEI/acrylate-coated (5PU, 6PU) and FEP-coated ®bres (2F, 3F), which were drawn under nearly the same conditions (Table 1), have maximum strengths, which are in the same range from about 400 to 500 MPa. On the other hand, the smaller strength zone of the curve is dominated by the ®bres with FEP. PEI/acrylate-coated ®bre 4PU demonstrated that the strength could be increased by increasing the drawing speed (Fig. 3(a), Table 2). Mean ®bre strengths now shifted to 620 and even 1000 MPa. The latter strength could be con®rmed by further bending tests, in which strengths of 920 and 1040 MPa were measured (the tests were
conducted at reduced platen velocities of 1 and 5 mm/min, respectively). Since the ®rst mechanical tests on ®bre 4PU had been carried out some time after drawing (Table 2), we investigated if the strength inhomogenities observed were due to an aging eect or had been induced during drawing. For this reason, further consecutive lengths of ®bres 4PU, and especially of ®bres 5PU and 6PU, were taken from time to time, and their strengths measured (Table 3). For completeness, Table 3 also provides data for ®bres 3F, 5P and 6U. It emerged that the strength of the dual coated ®bres did not change with time, hence, the strength variations, encountered in ®bres 5PU and 4PU, must have been present before ageing. Nor did that of ®bres 3F and 6U. However, decrease of strength did occur in ®bre 5P. Its mean strength decreased over ®ve months from initial 500 to 210 MPa. Inspecting the neckdown of preform 1 by SEM, all the surface ¯aws already described in [1±3] were observed. By contrast, the PEI-coated surface of preform 5 was smooth, but had several spots, in which the coating was torn open. No corrugation could be found on the glass surface beneath. Sur-
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
143
Fig. 3. (a) Weibull plots of ®bres 1, 2F, 3F, 4PU, 5PU and 6PU. The lines are drawn as guides for the eye. (b) Weibull plots of ®bres 5P, 6P and 6U. (For clearness, the graph includes again the curves of ®bres 1 and 6PU.) The lines are drawn as guides for the eye.
prisingly, even the bare neck-down zone of preform 6 appeared smooth and did not have corrugation nor islands.
Fig. 4. Mirror size vs fracture stress for ®bres 1, 4PU, 6PU and 6U. The curve is the least squares ®t of the function, mirror size A fracture stress, to the data shown in Fig. 1 (calibration curve 1). The correlation coecient is R 0:968.
4. Discussion How do the ®bres in this study compare with the literature reports? As for ®bres pulled under nitrogen only, bending strength data are scarce to our knowledge and are exclusively about bare ®bres. ZBLAL ®bres [16] settled in the range from 50 to 350 MPa, which is in agreement with our ®bre 1 whose strength spans 180 to 310 MPa. ZBLAN ®bres [4] were stronger with a strength spanning 300±800 MPa. This range and smaller m's make them comparable to PEI-coated ®bres 5P and 6P. Best ®bre 4PU can be compared to that described in [6], which had similar structure, i.e., a glass ®bre diameter of 130 lm and an epoxyacrylate coating of 60 lm thickness, and was drawn at a speed of 10 m/min. Its strength extended from 700 to 1200 MPa. However, that strength had been obtained only after treating the preform surface with active ¯uorine and drawing it under reactive NF3 =Ar atmosphere. The smooth glass surface seen in the neck-down zone of our PEI-coated preforms or on our ®bres can be explained by an axial compressive force, which the coating exerts on the ®bre/preform during cooling due to the mismatch in the thermal expansion
144
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
Table 3 Fracture strengths of the ®bres versus time Fibre no.
Time after drawinga (d)
Fracture strength, r0 b (GPa)
3F 4PU 5Pc
545 244 62 163 59 90 90 56 106 29 29 (end part)
0:45 0:01 0:89 0:03 0:40 0:01 0:21 0:01 0:57 0:01 0:63 0:01 0:62 0:01 0:57 0:01 0:41 0:01 0:37 0:01 0:40 0:01 0:39 0:01 0:39 0:01
4.6 0.2 5.7 0.3 2.8 0.1 4.4 0.7 7.0 0.3 8.1 0.6 10.3 0.4 10.3 0.4 12.7 0.7 15.5 0.6 11.4 1.1 14.3 1.0 14.8 0.8
0:37 0:01
11.0 0.6
5PU 6P 6PU
6Uc
61 61 29 96 29
Weibull parameter, m
a
Figures with an asterisk denote samples of 0.4 m length, which after being fractured in two halves were stored for some time (indicated by +) and then fractured again. b r0 corresponds to a failure probability of 63.2 %. c Fibre pieces taken randomly.
coecients between glass and coating. According to [18], the axial stress is rax E
a1 a2 T Ts ; R2 =r2 1 m
3
where E is the Young modulus of PEI (3300 MPa 1), a1 and a2 are the thermal expansion coecients of the coating (500 10 7 K 1 ) and the glass (194 10 7 K 1 for ZBLAN [19]), respectively, T is room temperature (298 K), Ts is the eective setting temperature (assumed to be 488 K, the Tg of the PEI coating), m is the Poisson ratio (assumed to be 0.3), and R and r are the radii of the coated ®bre and the glass ®bre, respectively. By taking the diameter of ®bre 5P (Table 2) and assuming a coating thickness of 0:5 lm, the axial stress becomes compressive, with rax being around )27 MPa. If the drawing of ®bres 5PU and 6PU is viewed chronologically, the bending strength will be seen to decrease from 630 to 470 MPa (5PU) and from
450 to 370 MPa (6PU). A decreasing compressive stress may account for these changes, since the PEI coatings of the preforms 5 and 6 decrease in drawing direction as well (Table 1). Due to the dierence in the PEI coating thickness (the layer of preform 5 is approximately half as thick as that of preform 6), the eect on the bending strength is larger in ®bre 5.
5. Conclusion By coating ¯uoride preforms with polyetherimide, ®bre drawing under nitrogen atmosphere is feasible. The drawing speed aected the bending strength of the ®bres. Optimisation of the dipcoating process will favour ®bre homogeneity. Further strength improvement by optimisation of the draw conditions should be possible.
Acknowledgements 1 General Electric Plastics Italia SpA, Viale Brianza 181, I20092 Cinisello Balsamo (MI), Italy, commercial product information.
The authors wish to thank R. DeFranceschi for SEM measurements.
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
References [1] S. Sakaguchi, Y. Terunuma, Y. Ohishi, T. Kanamori, J. Mater. Sci. Lett. 6 (1987) 1063. [2] P. McNamara, D.R. MacFarlane, Mater. Sci. Forum 32&33 (1988) 219. [3] G. Orcel, D. Biswas, J. Am. Ceram. Soc. 74 (1991) 1373. [4] H.W. Schneider, Mater. Sci. Forum 32&33 (1988) 561. [5] G.F. West, U.B. Unrau, Institut f ur Hochfrequenztechnik, Technical University of Braunschweig, Germany, Annual Report 1996, p. 59. [6] P.C. Pureza, P.H. Klein, W.I. Roberts, I.D. Aggarwal, J. Mater. Sci. 26 (1991) 5149. [7] M. Hartmann, G.H. Frischat, K. H ogerl, G.F. West, J. Non-Cryst. Solids 184 (1995) 209. [8] Y. Nishida, Y. Ohishi, K. Fuijura, T. Kanamori, S. Sudo, J. Am. Ceram. Soc. 80 (1997) 2495. [9] R. Gensler, C.J.G. Plummer, H.-K. Kausch, H. M unstedt, J. Mater. Sci. 32 (1997) 3037. [10] R. Torrecillas, A. Baudry, J. Dufay, B. Mortaigne, Polym. Degrad. Stab. 54 (1996) 267.
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[11] T. Katsuyama, H. Matsumura, in: E.R. Pike, W.T. Welford (Eds.), Infrared Optical Fibers, Adam Hilger, Bristol, 1989, p. 61. [12] M. Braglia, J. Kraus, S. Mosso, J. Non-Cryst. Solids 201 (1996) 237. [13] M. Braglia, C. Bruschi, E. Chierici, G. Dai, J. Kraus, S. Mosso, J. Non-Cryst. Solids 256&257 (1999) 220. [14] M.J. Matthewson, C.R. Kurkjian, S.T. Gulati, J. Am. Ceram. Soc. 69 (1986) 815. [15] R.H. Doremus, in: M. Tomozawa, R.H. Doremus (Eds.), Treatise on Materials Science and Technology, vol. 22, Glass III, Academic Press, New York, 1982, p. 170. [16] C.C.M. Wu, D. Lewis, K.R. McKinney, Proc. SPIE Int. Soc. Opt. Eng. 484 (1984) 78. [17] S. Sakaguchi, S. Mitachi, J. Am. Ceram. Soc. 66 (1983) C151. [18] D.A. Krohn, J. Amer. Ceram. Soc. 53 (1970) 505. [19] J.M. Parker, A.G. Clare, in: Proceedings of the Sixth International Symposium on Halide Glasses, ClausthalZellerfeld, Germany, 1989, p. 425.
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Thermoplastic polyetherimide ± a helpful aid for the drawing of ¯uoride ®bres J. Kraus *, C. Bruschi, E. Billi Agilent Technologies, Via G. Reiss Romoli 274, I-10148 Torino, Italy
Abstract A coating layer of thermoplastic polyetherimide (PEI) of less than 40 lm prevents ¯uoride preforms from moisture attack during the drawing process. If over-coated in-line with acrylate, the bending strength of the ®bres, which was seen to depend strongly on the drawing speed, reached 1 GPa even under nitrogen atmosphere. A correlation between ®bre strength and layer thickness was found. Furthermore, a strength comparison with ¯uoride ®bres coated with a ¯uorinated copolymer was made. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 42.81.Bm; 61.41.+e; 62.20.Mk
1. Introduction One of the problems in the drawing of ¯uoride ®bres is the formation of a corrugated skin on the surface of the preform neck-down in the hot zone of the drawing furnace (see micrographs in [1±3]). This skin, which was reported to be amorphous [2] as well as crystalline [4], may form as early as the preform head necks down to a gob to initiate ®bre drawing or during ®bre drawing. Generally, as drawing proceeds, it is torn o by the pulling force into small ¯oe-like islands scattered over the ®bre, which constitute one of the main ¯aw sources and aect the mechanical strength of the ®bre [1,5]. Moisture in the drawing atmosphere reacting with the preform and inducing hydrolysis was con®rmed to be the reason for its formation [3±5].
* Corresponding author. Tel.: +39-011 229 2447; fax: +39-011 229 2434. E-mail address: [email protected] (J. Kraus).
Hence, work on the fabrication of high-strength ¯uoride ®bres has been directed to keeping away or removing moisture from the preform surface. Control of the drawing atmosphere, special preform surface treatments and development of protective glass over-claddings or polymeric coatings were the main approaches undertaken ([3,4,6±8] and references therein). In this paper, we present our results on avoidance of skin formation by coating the preforms with thin layers of a thermoplastic polyetherimide (PEI) before drawing. This polymer, which is completely amorphous (Tg 215°C) and soluble [9], is stable to temperatures 490°C [10], hence greater than those normally employed for ¯uoride ®bre drawing [11]. The mechanical properties of the resulting ®bres, which for the most part were again over-coated in-line with an ultraviolet (UV)-curable resin, are compared to those of bare ®bres and ®bres coated with ¯uorinated ethylene±propylene copolymer (Te¯on FEP).
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 3 9 3 - 3
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J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
2. Experimental The nine ®bres examined were drawn from multimode preforms or clad rods of the ZBLYAN type. Multimode preforms consisted of Er3 doped core glasses of the ZBLYALi composition and of clad glasses of the ZBLYAN type [12,13]. All the preform surfaces were treated in the same manner by both mechanical polishing and chemical etching [13] and then covered by Te¯on FEP (P®ster) or PEI (Ultemâ 1000, General Electric Plastics Italia), respectively. In the former case, the glass preform was ®tted into a FEP tube, which was then ®tted into another FEP tube. In the latter, the preform was dipped at room temperature into a solution of 1 g of PEI in 50 ml of N ; N -dimethylacetamide, which was prepared previously by dissolving the dried polymer with stirring at 60°C and cooling to room temperature. After removing the preform from the solution, the solvent was evaporated under vacuum. This dip-coating step was repeated 5 or 10 times. The coated preform was mounted on the drawing tower, ¯ushed with nitrogen, which was used without further drying, for 2 h and then drawn into ®bre under nitrogen atmosphere. As for the heating system, a radiofrequency (RF) induction furnace was used. Only ®bres from dip-coated preforms were partially over-coated in-line with a UV-curable urethane acrylate resin (DSM Desotech, Desolite 3471-314). Table 1 summarises the preforms used and the drawing conditions employed.
Fibres from preforms 5 and 6 were stored in clean room conditions at 21°C and a relative humidity of 50%, all the other ®bres in laboratory atmosphere. Mechanical properties were determined from two-point bending tests (Lloyd Instruments LR5K) on samples of 0.2 m length by calculating the fracture stress by [14] r 1:198 E
d
df dc 2dg
1
with r, E, df, d, dc, 2dg being the fracture stress (GPa), the Young's modulus (50:4 0:2 GPa [4]), the glass ®bre diameter (lm), the distance between platen at ®bre fracture (lm), the overall ®bre diameter including any coating (lm), and the total depth of the two platen grooves (800 lm), respectively. If not stated otherwise, the platen velocity was 10 mm/min. Generally, the fracture stress was determined on consecutive ®bre lengths of 4±5 m, i.e., 20±25 samples, and the data treated by Weibull statistics. Furthermore, fracture stress data were corroborated by analysis of the fracture surface of the broken ®bres. As known [15], the distance, r, from the fracture origin, which is in the centre of a smooth region with circular or half-circular shape called the mirror, to the mist-hackle region outside the mirror, is related to the fracture stress by the empirical equation [15±17], p r M= r;
2
Table 1 Coated preforms and drawing conditions used
a
Preform no.
Preform con®guration
Preform coating (thickness (lm))
Drawing conditions: gas ¯ow (l/min) / neck-down temp. (°C) /drawing speed (m/min) /drawing tension (g)
1 2 3 4 5 6
Cladding rod Cladding rod Multimode, rod-in-tube Multimode, rod-in-tube Multimode, rotational casting Multimode, rod-in-tube
None FEP (1150) FEP (1150) PEIb (n.m.a ) PEI (24 6, 4:0 0:3)c PEI (41 3, 10)d
1.5 / 315 / 0.4 / 0.4 7.5 / 335 / 4.0 / 0.5 7.5 / 334 4 / 7.4 / n.m.a 7.5 / n.m.a / 16.3 / n.m.a 7:5=354 1=5:2 0:7=28 3 7:5=329 4=5:3 0:3=23 2
Not measured. Five layers. c Five layers; thickness measured with a pro®lometer (Alpha step 200 (Tecnor Instruments)) on both ends of the preform (®rst value refers to the preform head). d Ten layers over half the preform; thickness measured with a mechanical gauge (Tesamaster) on ®lms peeled o from both ends of the preform (®rst value refers to the preform head). b
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
141
whose mirror radii were measured by scanning electron microscopy (SEM). The slope gives M 1:215 MPa m1=2 , which is in agreement with M 1:24 0:04 MPa m1=2 as found in [16]. Since the measurement of mirror radii is much more convenient by optical microscopy rather than SEM but less precise, a second calibration curve was established (Fig. 2) to correlate the radii revealed by optical microscopy to those measured by SEM. The curve was valid for radii up to 20 lm. Larger radii were used as measured. 3. Results
Fig. 1. Mirror size vs fracture stress (calibration curve 1). Uncertainties are less than the size of the data points. The curve is a least squares ®t of the function, mirror size A fracture stress, to the data. The correlation coecient of the ®t is R 0:978.
where M, the mirror constant, depends on the material [15]. p Fig. 1 shows the calibration curve in which 1= r is plotted against r for 17 ®bres,
Fig. 2. Mirror radius by SEM vs mirror radius by optical microscope (calibration curve 2). The data are ®tted best by a second-order polynomial (y 0:538x 0:024x2 ). The correlation coecient of the ®t is R 0:990.
In view of the large dierence in the coating thickness of FEP-coated preforms to PEI-coated preforms (Table 1), we decided to keep the gas ¯ow constant rather than the neck-down temperature during drawing. Temperature pro®le measurements made by us previously on both FEP-coated and bare multimode preforms in the proximity of the neck-down zone had shown that, only in case of FEP, a radial temperature dierence of up to 20° between the clad surface and the core/clad interface can occur. The gas ¯ow rate was set for stable drawing with no undercooling of the neck-down zone. Acceptable ®bre-diameter ¯uctuations could be expected. This process worked well for all the preforms except for bare preform 1, whose ¯ow rate had to be reduced to 1/5 of the set ¯ow rate. Preform 6 was only partly covered with PEI to obtain a maximum of ®bre variety, i.e. ®bres with PEI, ®bres with PEI and acrylate, and ®bres without PEI but with acrylate, from the same preform. Table 2 gives an overview of the ®bres obtained in terms of ®bre geometry, initial fracture strength and Weibull distribution parameters (m's), and Fig. 3(a) and (b) present their Weibull plots. Fig. 4 shows the corresponding plots of fracture strength vs mirror size for some of the ®bres. As expected, bare ®bre 1 had the smallest strength and the largest diameter ¯uctuations of all the ®bres fabricated. Continuous ®bre drawing was quite dicult, since the ®bre broke repeatedly during its winding on the take-up drum. By contrast, PEI-coated ®bres 5P and 6P, whose
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Table 2 Geometry, initial fracture strength and Weibull parameters of the ¯uoride ®bres drawn Fibre noa
Fibre length investigated (m)
Mean glass ®bre diameterb (lm)
Mean overall ®bre diameterb (lm)
Fracture strength, r0 c (GPa) (samples)
Weibull parameter, m
1 2F 3F 4PU
3 5 15 22d
151 25 121 4 125 4 149 8
151 25 149 5 154 6 200 5
5P 5PU 6P 6PU 6U
10 19g 7 28h 14
143 20 177 9 130 15 135 7 143 11
143 20 227 5 130 15 207 5 216 7
0:26 0:01 0:47 0:01 0:45 0:01 0:62 0:01 1:00 0:03 0:50 0:01 0:47 0:01 0:58 0:01 0:45 0:01 0:37 0:01
8:0 0:6 5:2 0:4 3:3 0:3 11:5 0:7 6:3 0:2 2:9 0:2 6:7 0:4 2:6 0:2 15:7 0:7 12:1 0:6
(14) (24) (24) (24)e (19)f (20) (23) (20) (25) (25)
a Preform numbers and ®bre numbers are corresponding. Since from preforms 5 and 6 several ®bres with dierent coatings have been obtained, the letters F, P and U after the numerals indicate inner and outer coating and stand for Te¯on FEP, PEI, and urethane acrylate, respectively. b Data from all the bending tests; the ®bre length is given in column 2. c r0 corresponds to a failure probability of 63.2%. If not stated otherwise, the bending tests were carried out the day after drawing. d Total ®bre length 50 m. e Fibre ®rst measured after 30 days. f Measured after 97 days. g The ®bre was investigated from the end of drawing versus the beginning. h The ®bre was investigated from the beginning of drawing versus the end (total ®bre length 100 m).
coating thickness was less than 1 lm, could be pulled continuously without problems, had less diameter variations than ®bre 1 and had the largest strength range extending to 800 MPa (Fig. 3(b)). Consequently, their ms were smallest. If these ®bres were coated in-line with acrylate, the ms would increase up to ®ve times as much (®bre 6PU) and the mean bending strength was 450 MPa. The m remained large even after the transition from dual PEI/acrylate-coating to single acrylate coating (®bre 6U), whilst the strength decreased by 80 MPa. Fig. 3(a) shows that both PEI/acrylate-coated (5PU, 6PU) and FEP-coated ®bres (2F, 3F), which were drawn under nearly the same conditions (Table 1), have maximum strengths, which are in the same range from about 400 to 500 MPa. On the other hand, the smaller strength zone of the curve is dominated by the ®bres with FEP. PEI/acrylate-coated ®bre 4PU demonstrated that the strength could be increased by increasing the drawing speed (Fig. 3(a), Table 2). Mean ®bre strengths now shifted to 620 and even 1000 MPa. The latter strength could be con®rmed by further bending tests, in which strengths of 920 and 1040 MPa were measured (the tests were
conducted at reduced platen velocities of 1 and 5 mm/min, respectively). Since the ®rst mechanical tests on ®bre 4PU had been carried out some time after drawing (Table 2), we investigated if the strength inhomogenities observed were due to an aging eect or had been induced during drawing. For this reason, further consecutive lengths of ®bres 4PU, and especially of ®bres 5PU and 6PU, were taken from time to time, and their strengths measured (Table 3). For completeness, Table 3 also provides data for ®bres 3F, 5P and 6U. It emerged that the strength of the dual coated ®bres did not change with time, hence, the strength variations, encountered in ®bres 5PU and 4PU, must have been present before ageing. Nor did that of ®bres 3F and 6U. However, decrease of strength did occur in ®bre 5P. Its mean strength decreased over ®ve months from initial 500 to 210 MPa. Inspecting the neckdown of preform 1 by SEM, all the surface ¯aws already described in [1±3] were observed. By contrast, the PEI-coated surface of preform 5 was smooth, but had several spots, in which the coating was torn open. No corrugation could be found on the glass surface beneath. Sur-
J. Kraus et al. / Journal of Non-Crystalline Solids 284 (2001) 139±145
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Fig. 3. (a) Weibull plots of ®bres 1, 2F, 3F, 4PU, 5PU and 6PU. The lines are drawn as guides for the eye. (b) Weibull plots of ®bres 5P, 6P and 6U. (For clearness, the graph includes again the curves of ®bres 1 and 6PU.) The lines are drawn as guides for the eye.
prisingly, even the bare neck-down zone of preform 6 appeared smooth and did not have corrugation nor islands.
Fig. 4. Mirror size vs fracture stress for ®bres 1, 4PU, 6PU and 6U. The curve is the least squares ®t of the function, mirror size A fracture stress, to the data shown in Fig. 1 (calibration curve 1). The correlation coecient is R 0:968.
4. Discussion How do the ®bres in this study compare with the literature reports? As for ®bres pulled under nitrogen only, bending strength data are scarce to our knowledge and are exclusively about bare ®bres. ZBLAL ®bres [16] settled in the range from 50 to 350 MPa, which is in agreement with our ®bre 1 whose strength spans 180 to 310 MPa. ZBLAN ®bres [4] were stronger with a strength spanning 300±800 MPa. This range and smaller m's make them comparable to PEI-coated ®bres 5P and 6P. Best ®bre 4PU can be compared to that described in [6], which had similar structure, i.e., a glass ®bre diameter of 130 lm and an epoxyacrylate coating of 60 lm thickness, and was drawn at a speed of 10 m/min. Its strength extended from 700 to 1200 MPa. However, that strength had been obtained only after treating the preform surface with active ¯uorine and drawing it under reactive NF3 =Ar atmosphere. The smooth glass surface seen in the neck-down zone of our PEI-coated preforms or on our ®bres can be explained by an axial compressive force, which the coating exerts on the ®bre/preform during cooling due to the mismatch in the thermal expansion
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Table 3 Fracture strengths of the ®bres versus time Fibre no.
Time after drawinga (d)
Fracture strength, r0 b (GPa)
3F 4PU 5Pc
545 244 62 163 59 90 90 56 106 29 29 (end part)
0:45 0:01 0:89 0:03 0:40 0:01 0:21 0:01 0:57 0:01 0:63 0:01 0:62 0:01 0:57 0:01 0:41 0:01 0:37 0:01 0:40 0:01 0:39 0:01 0:39 0:01
4.6 0.2 5.7 0.3 2.8 0.1 4.4 0.7 7.0 0.3 8.1 0.6 10.3 0.4 10.3 0.4 12.7 0.7 15.5 0.6 11.4 1.1 14.3 1.0 14.8 0.8
0:37 0:01
11.0 0.6
5PU 6P 6PU
6Uc
61 61 29 96 29
Weibull parameter, m
a
Figures with an asterisk denote samples of 0.4 m length, which after being fractured in two halves were stored for some time (indicated by +) and then fractured again. b r0 corresponds to a failure probability of 63.2 %. c Fibre pieces taken randomly.
coecients between glass and coating. According to [18], the axial stress is rax E
a1 a2 T Ts ; R2 =r2 1 m
3
where E is the Young modulus of PEI (3300 MPa 1), a1 and a2 are the thermal expansion coecients of the coating (500 10 7 K 1 ) and the glass (194 10 7 K 1 for ZBLAN [19]), respectively, T is room temperature (298 K), Ts is the eective setting temperature (assumed to be 488 K, the Tg of the PEI coating), m is the Poisson ratio (assumed to be 0.3), and R and r are the radii of the coated ®bre and the glass ®bre, respectively. By taking the diameter of ®bre 5P (Table 2) and assuming a coating thickness of 0:5 lm, the axial stress becomes compressive, with rax being around )27 MPa. If the drawing of ®bres 5PU and 6PU is viewed chronologically, the bending strength will be seen to decrease from 630 to 470 MPa (5PU) and from
450 to 370 MPa (6PU). A decreasing compressive stress may account for these changes, since the PEI coatings of the preforms 5 and 6 decrease in drawing direction as well (Table 1). Due to the dierence in the PEI coating thickness (the layer of preform 5 is approximately half as thick as that of preform 6), the eect on the bending strength is larger in ®bre 5.
5. Conclusion By coating ¯uoride preforms with polyetherimide, ®bre drawing under nitrogen atmosphere is feasible. The drawing speed aected the bending strength of the ®bres. Optimisation of the dipcoating process will favour ®bre homogeneity. Further strength improvement by optimisation of the draw conditions should be possible.
Acknowledgements 1 General Electric Plastics Italia SpA, Viale Brianza 181, I20092 Cinisello Balsamo (MI), Italy, commercial product information.
The authors wish to thank R. DeFranceschi for SEM measurements.
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References [1] S. Sakaguchi, Y. Terunuma, Y. Ohishi, T. Kanamori, J. Mater. Sci. Lett. 6 (1987) 1063. [2] P. McNamara, D.R. MacFarlane, Mater. Sci. Forum 32&33 (1988) 219. [3] G. Orcel, D. Biswas, J. Am. Ceram. Soc. 74 (1991) 1373. [4] H.W. Schneider, Mater. Sci. Forum 32&33 (1988) 561. [5] G.F. West, U.B. Unrau, Institut f ur Hochfrequenztechnik, Technical University of Braunschweig, Germany, Annual Report 1996, p. 59. [6] P.C. Pureza, P.H. Klein, W.I. Roberts, I.D. Aggarwal, J. Mater. Sci. 26 (1991) 5149. [7] M. Hartmann, G.H. Frischat, K. H ogerl, G.F. West, J. Non-Cryst. Solids 184 (1995) 209. [8] Y. Nishida, Y. Ohishi, K. Fuijura, T. Kanamori, S. Sudo, J. Am. Ceram. Soc. 80 (1997) 2495. [9] R. Gensler, C.J.G. Plummer, H.-K. Kausch, H. M unstedt, J. Mater. Sci. 32 (1997) 3037. [10] R. Torrecillas, A. Baudry, J. Dufay, B. Mortaigne, Polym. Degrad. Stab. 54 (1996) 267.
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[11] T. Katsuyama, H. Matsumura, in: E.R. Pike, W.T. Welford (Eds.), Infrared Optical Fibers, Adam Hilger, Bristol, 1989, p. 61. [12] M. Braglia, J. Kraus, S. Mosso, J. Non-Cryst. Solids 201 (1996) 237. [13] M. Braglia, C. Bruschi, E. Chierici, G. Dai, J. Kraus, S. Mosso, J. Non-Cryst. Solids 256&257 (1999) 220. [14] M.J. Matthewson, C.R. Kurkjian, S.T. Gulati, J. Am. Ceram. Soc. 69 (1986) 815. [15] R.H. Doremus, in: M. Tomozawa, R.H. Doremus (Eds.), Treatise on Materials Science and Technology, vol. 22, Glass III, Academic Press, New York, 1982, p. 170. [16] C.C.M. Wu, D. Lewis, K.R. McKinney, Proc. SPIE Int. Soc. Opt. Eng. 484 (1984) 78. [17] S. Sakaguchi, S. Mitachi, J. Am. Ceram. Soc. 66 (1983) C151. [18] D.A. Krohn, J. Amer. Ceram. Soc. 53 (1970) 505. [19] J.M. Parker, A.G. Clare, in: Proceedings of the Sixth International Symposium on Halide Glasses, ClausthalZellerfeld, Germany, 1989, p. 425.