FIBRE FORMING PROCESS

Abstract

A fibre drawing method for forming a fibre from a low Young's modulus material.

Claims

1. A fibre drawing method for forming a fibre from a low Young's modulus material, the method including: feeding a low Young's modulus fibre preform into a draw zone of a fibre drawing apparatus, and drawing a fibre from a low Young's modulus material preform at a sufficiently low tension to maintain a neck down region of the low Young's modulus material preform at a neck down position within the draw zone.

2. The method of claim 1, wherein the step of drawing a fibre from the low Young's modulus material preform is performed under a normalised tension of 50 gm-f/mm2 or less.

3. A fibre drawing method for forming a fibre from a low Young's modulus material, the method including: feeding a low Young's modulus fibre preform into a draw zone of a fibre drawing apparatus; and drawing a fibre from the low Young's modulus material preform under a normalised tension of 50 gm-f/mm2 or less.

4. The method of claim 3, wherein the normalised tension is 40 gm-f/mm2 or less.

5. The method of claim 4, wherein the normalised tension is 25 gm-f/mm2 or less.

6. The method of claim 3, wherein the method includes maintaining a neck down region of the low Young's modulus material preform at a neck down position within the draw zone.

7. The method of claim 1, wherein the low Young's modulus material has a Young's modulus of 300 MPa or less at room temperature.

8. The method of claim 7, wherein the low Young's modulus material has a Young's modulus of 100 MPa or less.

9. The method of claim 8, wherein the low Young's modulus material has a Young's modulus of 10 MPa or less.

10. The method of claim 1, wherein the low Young's modulus material is drawn at a draw temperature such that the low Young's modulus material is viscous.

11. The method of claim 10, wherein the draw temperature is above the glass transition temperature of the low Young's modulus material.

12. The method of claim 1, wherein the low Young's modulus material is selected from the group consisting of: a polymer, rubber, silicone compound, or hydrogel.

13. The method of claim 12, wherein the low Young's modulus material is a polymer, and the polymer is a thermoplastic elastomer selected from the group consisting of: polyurethane, poly(styrene-b-(ethylene-co-butylene)-b-styrene), and polystyrene-polyisoprene triblock copolymer.

14. The method of claim 1, wherein the step of drawing the fibre has a draw error within the range of ±20% of a set point diameter of the fibre.

15. The method of claim 14, wherein the draw error is within the range of ±10% of the set point diameter of the fibre.

16. The method of claim 1, wherein the low Young's modulus fibre preform has a hollow core structure, and prior to the step of feeding the low Young's modulus material to the draw zone, the method initially includes: forming a paraxially aligned arrangement of a plurality of hollow tubes of a low Young's modulus material in a configuration corresponding to the hollow core structure; heating the plurality of tubes to a temperature above the glass transition temperature of the low Young's modulus material; and baking the plurality of tubes at the temperature for a time sufficient to sinter the plurality of tubes, and form the low Young's modulus fibre preform.

17. The method of claim 1, wherein the step of drawing the fibre includes drawing the fibre to a fibre diameter of less than 1 mm.

18. A fibre formed from a low Young's modulus material according to the method of claim 1.

19. The method according to claim 1 wherein the fibre is an optical fibre.

20. An optical fibre sensor device including an optical fibre formed from a low Young's modulus material according to the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG. 1: graph showing (a) the draw tension and (b) the normalised draw tension of polyurethane of different Inner to Outer Diameter ratio for different furnace temperatures.

[0044] FIG. 2: graph showing (a) the draw tension and (b) the normalised draw tension of PMMA of different Inner to Outer Diameter ratio for different furnace temperatures.

[0045] FIG. 3: graph showing diameter error at different normalised tensions for polyurethane of different ID/OD.

[0046] FIG. 4: graph showing diameter error at different normalised tensions for PMMA of different ID/OD.

[0047] FIG. 5: graph comparing diameter error at different normalized tensions for polyurethane and PMMA.

[0048] FIG. 6: graph showing draw tension for polyurethane at different feed rates.

[0049] FIG. 7: graph showing diameter error for polyurethane at different feed rates

[0050] FIG. 8: Schematic of a polyurethane fibre (a) without jacket and (b) with jacket.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0051] The present invention relates to a method of fibre drawing for fabricating low Young's modulus (YM) materials into fibre. To an experienced fibre-drawing expert, low YM materials are incompatible with current fibre drawing techniques. However, the inventors have now found that fibre drawing processes may be used to form fibres from low YM materials by adopting low draw tensions to maintain the neck down position within a draw zone of a fibre drawing apparatus.

[0052] Whilst the method of the invention may be applied to a range of different types of low YM fibres, the invention will now be described generally with reference to microstructured optical fibres (MOFs). A basic hollow core structure of a MOF is shown in FIG. 8. Briefly, MOFs are formed using a process in which a guiding mechanism is determined by a pattern of air holes in the resultant fibre. The most popular method of drawing microstructured fibres is the “stack-and-draw” method, where tubes are stacked within a larger outer jacket tube. These individual tubes are fused together in the drawing furnace at the time of drawing.

[0053] The first step of drawing the fibre is to put the preform into a draw zone (e.g. a furnace or hot zone) of a draw tower. In general, the preforms made with high YM materials are put into the furnace of the draw tower and allowed some time to heat to the drawing temperature. This stage is generally known as a “preheat stage” which allows the high YM material to soften before drawing commences. In comparison, a low YM preform can be stretched at room temperature, which could cause both elastic of plastic deformation depending on the applied tension. Therefore, for low YM materials a preheat stage may not be required in some instances, e.g. for some low YM materials the preform is fed into the draw zone at an ideal drawing temperature and starts drawing immediately.

[0054] A very wide range of simple or complicated microstructures with low YM materials can be fabricated using the methods of the invention. By way of example, the inventors have prepared microstructures with different numbers of air holes, anti-resonant structures, and structure with metal wires inside the holes, such as those disclosed in A. Stefani, R. Lwin, B. T. Kuhlmey, and S. C. Fleming, “OAM generation, tunable metamaterials and sensors with highly deformable fibers,” in Advanced Photonics 2018 (BGPP, IPR, NP, NOMA, Sensors, Networks, SPPCom, SOF), OSA Technical Digest (online) (Optical Society of America, 2018), paper number Th1D.2 (the entire contents of which are hereby incorporated by reference). The present invention may be adapted to form low YM fibres possessing complicated microstructures such as these using the methods of the invention.

[0055] The methods of the invention may be applied to draw a diverse range of low YM materials into fibres, a non-limiting disclosure of suitable low YM materials include: polyurethane, poly(styrene-b-(ethylene-co-butylene)-b-styrene), and polystyrene-polyisoprene triblock copolymer.

EXAMPLES

Example 1

[0056] Draw Tension for Polyurethane and PMMA of Different Inner Diameter to Outer Diameter Ratio (ID/OD) at Different Furnace Temperatures

[0057] Polyurethane (a low YM material of 2-30 MPa) and PMMA (a high YM material of 2-5 GPa) preforms of different ID/OD were drawn to fibre at different furnace temperatures to observe their effects on draw tension. All preforms were of 6 mm outer diameter. Different inner diameters of 3 mm, 1.5 mm and 0.75 mm were used to create preforms with different ID/OD of 0.5, 0.25 and 0.125 respectively. All fibre draws had a fixed feed rate of 10 mm/min, with furnace temperature varying between 205-230° C. for polyurethane and 170-250° C. for PMMA. The final target fibre diameter was 300 μm. The draw tension was monitored as the furnace temperature was changed. FIG. 1 and FIG. 2 shows the effects furnace temperature has on draw tension for different ID/OD polyurethane and PMMA respectively. The normalised tensions against their ID/OD are shown as well.

[0058] From FIG. 1 and FIG. 2, it can be seen that the Draw Tension in all cases increases as furnace temperature decreases as expected. Also lower ID/OD leads to a lower draw tension at each set furnace temperature since there is less bulk cross-section material to heat and stretch. The draw tensions were normalized against their ID/OD in order to compare the draw tension for both materials. It can be seen that a distinct trend is achieved irrespective of their ID/OD, indicating the draw tensions are dependent on the bulk material characteristics.

[0059] Diameter Error for Different ID/OD at Different Tensions

[0060] FIG. 1 and FIG. 2 showed that fibres can be drawn at a wide range of furnace temperatures for a given feed rate and starting preform diameter. However the final target fibre diameter also has to be consistent. Variations greater than ±10% from the final target fibre diameter are generally considered unacceptable, and indicate an inability to maintain structural integrity of the internal structure of complex multi-hole fibres. These variations can be attributed to the following reasons: not achieving steady state conditions, draw tensions too high that preform cannot be drawn to fibre leading to snapping, or draw tensions too low causing slumping due to fibre drawing faster than the prescribed draw rate.

[0061] FIG. 3 and FIG. 4 show the diameter error at different normalized tensions for both polyurethane and PMMA of different ID/OD respectively. As expected, the errors increase as tension increases due to the material being stiffer and hence more difficult to pull and maintain diameter leading to snapping. A dotted line at 10% diameter error is added to indicate the normalised tensions that still achieve consistent diameters. For both polyurethane and PMMA, the diameter error is dependent on the material properties rather than the ID/OD. An exploded view of the diameter error against normalized tension up to 200 gm-f/mm.sup.2 for both polyurethane and PMMA is shown in FIG. 5. It shows opposite trends for each material whereby PMMA produces acceptable diameter error for normalized tensions above 50 gm-f/mm.sup.2, and generally improves the larger the tension, while polyurethane is better for normalized tension below 50 gm-f/mm.sup.2, and generally improves for lower tensions. Furthermore, for normalized tensions below 50 gm-f/mm.sup.2, PMMA suffers from fibre slumping, whereby the speed of the drawn material is moving faster than the draw rate.

[0062] Effects of Different Feed Rate

[0063] The rate the preform enters the furnace also affects the draw tension as it changes the amount of time the neck down is allowed to heat to drawing temperature. The polyurethane preforms were fed at 5, 10 and 15 mm/m in into the furnace at different temperatures. All the preforms had an outer diameter of 6 mm and inner diameter of 0.75 mm (ID/OD 0.125) and were drawn down to a final target diameter of 500 μm. FIG. 6 shows the draw tension greatly affected by the feed rate, whereby the faster feed rate shifts the required temperature to a higher range since the neck down has less time to heat up. FIG. 7 shows the diameter error from target 500 μm. Once again, like the results in FIG. 3, the diameter error is greatly dependent on the material's tension rather than the feed rate used. Also the draw conditions with normalised tensions below 50 gm-f/mm.sup.2 achieve acceptable diameter consistency.

[0064] Discussion

[0065] Polyurethane and PMMA preforms can be drawn to fibre under a variety of draw conditions. However there are only small windows of ideal conditions that will guarantee the production of successful fibre of consistent diameter. Firstly the inventors observed that the material properties (i.e. Young's modulus) for both polyurethane and PMMA are more important to the draw tension compared to the ID/OD of the preform. It only requires changes in the furnace temperature to compensate for differences in ID/OD. Secondly, of the range of normalised tension achieved, only specific regimes are suitable for achieving consistent diameter fibre. In the case of PMMA a range between 50-200 gm-f/mm.sup.2 is suitable, while for polyurethane it is restricted to extremely low tension of under 50 gm-f/mm.sup.2. Overall this can be attributed to polyurethane's inherent low Young's modulus that leads to draw conditions that cannot be achieved with PMMA.

Example 2

[0066] This example reports the drawing of a polyurethane preform with a hollow core structure as shown in FIG. 8 in comparison with a PMMA preform having a similar structure.

[0067] In this example, prior to drawing the low YM fibre, the inventors additionally subjected the low YM preform to a pre-treatment step in which individual tubes of the low YM material that form the hollow core structure are subjected to a heat treatment step where the tubes are bundled together and annealed in an oven to fuse the interface between the tubes. This is important to ensure the structure holds together during the drawing process. Practically, this is an important step to successful fabrication of low YM fibres. An additional benefit of this process is that it allows creation of jacketless fibres from low YM materials, which is potentially important for applications that require more sensitivity as this fibre structure can be more easily deformed by external perturbations.

[0068] The pre-treatment step included arranging the polyurethane tubes into the hollow core structure, and then baking this structure in an oven ˜140° C. for half an hour. This temperature was chosen as it is significantly higher than the glass transition temperature of polyurethane, allowing the adjacent polyurethane tubes to adhere together rapidly. Notwithstanding this, the skilled addressee will appreciate that the choice of oven temperature and duration are dependent on the nature of the low YM material, and whether the interface between the tubes adheres together without deforming them. This method for fabricating preforms can also be used for other materials, including high YM materials such as PMMA.

[0069] Polyurethane and PMMA preforms were drawn from a preform in a draw tower. The furnace temperature, draw tensions, and results are summarised in Table 1 below.

TABLE-US-00001 TABLE 1 Polyurethane and PMMA draw conditions Approximate Furnace Draw Normalised Temp Tension Draw Tension (° C.) (gm-f) (gm-f/mm.sup.2) Results Polyurethane fibre draw conditions 208 30 204 High levels of diameter fluctuations (±50%) and then snap 211 15 101 Diameter fluctuations (±20%) 215 6 20 Consistent diameter 220 2 13 Consistent diameter 225 1 6 Fibre slump PMMA fibre draw conditions 171 285 1935 High levels of diameter fluctuations (±50%) and then snap 185 138 937 Diameter fluctuations (±20%) 202 65 441 Consistent diameter 210 30 203 Consistent diameter 223 15 101 Fibre slump 235 7 47 Fibre slump 255 3 20 Fibre slump

[0070] The results show that polyurethane preform can be successfully drawn at a constant tension below 6 gm-f. The resulting structures were maintained over lengths in excess of 10 m. Fibres were drawn to an external diameter of 500 μm, with a fibre diameter uniformity of ±10 μm. For comparative purposes, the PMMA fibre required >30 gm-f of draw tension.

[0071] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.