Patent ID: 12257793

DETAILED DESCRIPTION OF THE EMBODIMENTS

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.

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 inFIG.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.

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.

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.

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

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

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.1andFIG.2shows 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.

FromFIG.1andFIG.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.

Diameter Error for Different ID/OD at Different Tensions

FIG.1andFIG.2showed 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.

FIG.3andFIG.4show 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/mm2for both polyurethane and PMMA is shown inFIG.5. It shows opposite trends for each material whereby PMMA produces acceptable diameter error for normalized tensions above 50 gm-f/mm2, and generally improves the larger the tension, while polyurethane is better for normalized tension below 50 gm-f/mm2, and generally improves for lower tensions. Furthermore, for normalized tensions below 50 gm-f/mm2, PMMA suffers from fibre slumping, whereby the speed of the drawn material is moving faster than the draw rate.

Effects of Different Feed Rate

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.6shows 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.7shows the diameter error from target 500 μm. Once again, like the results inFIG.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/mm2achieve acceptable diameter consistency.

Discussion

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/mm2is suitable, while for polyurethane it is restricted to extremely low tension of under 50 gm-f/mm2. 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

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

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.

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.

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 1Polyurethane and PMMA draw conditionsApproximateFurnaceDrawNormalisedTempTensionDraw Tension(° C.)(gm-f)(gm-f/mm2)ResultsPolyurethane fibre draw conditions20830204High levels of diameterfluctuations (±50%) and thensnap21115101Diameter fluctuations (±20%)215620Consistent diameter220213Consistent diameter22516Fibre slumpPMMA fibre draw conditions1712851935High levels of diameterfluctuations (±50%) and thensnap185138937Diameter fluctuations (±20%)20265441Consistent diameter21030203Consistent diameter22315101Fibre slump235747Fibre slump255320Fibre slump

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.

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.