Negative Poisson’s Ratio Polyester Fibers

Negative Poisson’s Ratio Polyester Fibers

Ravirala, Naveen

Abstract Auxetic materials arc referred to as those having negative Poisson’s ratio (v). Initial work at Bolton successfully fabricated auxctic polypropylene fiber using a novel thermal meltspinning technique. This paper reports in detail both the methods and principles involved in screening polyester powder and also the manufacturing method for successful production of auxctic polyester fibers. Videoextensometry along with micro-tensile testing were used to measure the Poisson’s ratio of the fiber. The Poisson’s ratio of the polyester fiber was found to vary between -U.65 and -0.75.

Key words auxetic, negative Poisson’s ratio, polyester fiber, videoextensometry

Auxetic materials when stretched axially expand instead of contracting laterally. Following Lakes’s [1] successful production of auxetic foams, a variety of auxetic products have been fabricated including honeycombs [2], polymeric and metallic foams [1] and microporous polymers [3, 4]. An interesting feature of auxetic materials is that they are predicted [5], and have been found, to have enhanced properties. For example, it has been shown experimentally that the indentation resistance [6] of auxetic materials has been enhanced by up to four times when compared with the conventional equivalent. Other enhanced properties include plane strain fracture toughness [7], energy absorption [8], and shear modulus [9].

The first synthetic auxetic microporous polymer was a particular form of polytetrafluoroethylene (PTFE) [10]. It was found that the auxeticity in this case was solely because of its complex microstructure [10]. This consisted of nodules interconnected by fibrils that react co-operatively to produce a negative Poisson’s ratio. Similar microstructures have also been engineered in polymers such as ultra-high-molecular-weight polyethylene (UHMWPE) [3], polypropylene [4] and nylon [11], which were all produced by a novel thermal processing route consisting of three distinct stages; compaction [12], sintering [13] and ram extrusion [14]. The entire process takes place in a specially designed extrusion rig with extrudates produced in the form of cylindrical rods. Although having this special property, there were found to be limitations on the production of auxetic materials in the form of cylinders. In particular, the cylinders (having diameters varying between 9 and 15 mm) were found to be unsuitable for application-based research and were restricted to laboratory-based testing. Moreover, the process involved was not continuous and problems were envisaged in producing them on a large scale.

More recently, a novel thermal processing techniqueinvolving melt spinning has been employed to produce an auxetic product in a more useful and usable form, namely as a fiber [15]. This has led to auxetic polypropylene (PP) fibers being fabricated in a continuous process. Videoextensometry analysis was used to show the auxetic nature of the PP fiber. The processing route developed for auxetic polypropylene fibers is, in principle, flexible enough to be adapted to produce other polymeric fibers [15] and films [16] in auxetic form.

This paper reports in detail the production of auxetic polyester fiber, including the characterization by videoextensometry.

Experimental Methods

Preliminary Characterization of Polyester Granules

The polyester resin used in the fabrication of auxetic polyester fiber was poly (trimethyleneterephthalate [17] or 3GT) supplied by DuPont in the form of granules. In order to establish the thermal processing window, it was necessary to obtain the thermal characteristics of the polyester granules using differential scanning calorimetry (DSC). The DSC was carried out by heating a small amount of polyester sample (5.5 mg) under flowing nitrogen at 10 mL min^sup -1^. The heating rate was maintained at 10°C/minute and the temperature profile ranged from ambient (25°C) to 310°C. Figure 1 shows the DSC curve displaying the onset temperature (the temperature at which the polymer just starts melting) at 205°C, and the peak melting point at 229°C. From previous results [18] this indicated that the processing window would occur at a particular temperature between the regions of onset temperature and peak melting point. Therefore, the processing temperature window was defined to be 210 and 230°C. Following the PP work, a flat profile across all the zones of the extruder was employed.

Polyester Powder Production

The previously reported auxetic PP fibers were produced from finely divided powder particles. Thus there was a need to obtain powder particles from the supplied polyester granules prior to extrusion. The polyester granules were subjected to in-house cryogenic grinding using liquid nitrogen. Ground polyester particles of less than 150 µm were collected to carry out the extrusion.

The 3GT granules and powder required pre-drying to avoid hydrolysis during extrusion. Therefore, 3GT granules (used during purging) along with 3GT powder were predried under a partial vacuum oven at 108°C temperature for 2 days before extrusion.

Scanning electron microscopy (SEM) was employed to characterize the polyester powder derived from grinding. Typically observations were made at approximately 200x magnification. The micrographs of polyester powder were studied in depth not only to analyse the size distributions but also to understand the surface roughness and shape. UTHSCSA Image Tool v3 [19] was employed to analyse the powder size distribution and the aspect ratio (calculated as the ratio of the major axis to minor axis) using the micrographs of polyester powder obtained from SEM.

Extrusion of 3GT Polyester Fibers

An initial extrusion experiment was carried out with 3GT granules to observe the behavior of the polymer during extrusion. The melt spinning process was performed using an Emerson and Renwick Ltd Labline extruder which consists of a 25.4 mm screw diameter, 3 : 1 compression ratio, length/diameter 24 : 1, five temperature zones each having individual thermostatic controls, and a die slot with 40-filament die having each hole size of 550 µm (Figure 2). The processing temperatures investigated were based on thermal analysis studies and were carried out at intervals of 5°C, leading to flat temperature profiles in the range of 230 to 210°C with screw speeds of 0.525 rad/s and take-up speed 0.075 m/s. The extrusion was carried out at higher temperature (230°C) profile and gradually decreased the temperature to 210°C until the viscosity of the powder bulk was too high to allow free flow through the die-zone. The polyester powder was fed through a hopper into the barrel and transferred through the barrel zones to the die zone, thereby undergoing sintering due to the maintained temperatures along the zones of the extruder. The fibers were extruded from the die head and cooled before winding.

Characterization of Extruded Fibers

Figure 3 shows the load-elongation data of a typical fiber. Samples were tested for auxeticity by deforming them to 1% strain, thus not exceeding the elastic limit of the fibers. Characterization of the extruded fibers was carried out by using videoextensometry and microtensile testing equipment. Figure 4 shows a schematic of the videoextensometer and microtensile testing stage. The camera of the videoextensometer was mounted vertically above the microtensile stage to capture the image of the fiber. The PC, attached to the microtensile unit processes the load-extension data from the microtensile stage fitted with a 77 N load cell. The PC^sub 2^ attached to the videoextensometer records the changes in length and width of the fiber during the test. Polyester fibers cut to lengths of approximately 10 mm, marked with two small markers at a distance of approximately 1 mm apart along the length of the fiber, were mounted between the two clamps of the microtensile tester. Figure 5 shows a typical sample with the fiducial markers attached. Each of the tests included mounting the sample between the jaws of the microtensile stage and cyclically applying and removing a load (up to four cycles). A strain rate of 0.1 mm/minute was employed with load applied until a maximum strain of 1% was reached. The videoextensometer operates directly as a strain meter by tracking the displacement of the edges of the sample as defined by a contrast change in the video image at the edges of the attached targets (length) or fiber edges (width) caused by mechanical strain to the specimen. During the test, the marked length was divided into ten regions by the videoextensometer and the individual width of each was recorded.

Results

Polyester Powder Characterization

Figure 6 shows a SEM micrograph of the ground polyester powder. The aspect ratio of the powdered particles was found to vary between 1.5 and 2, with a typical particle dimension of 50 µm in the minor axis and 100 µm in the major axis. It was also observed that the particles had a rough surface. Hence, this particular polyester powder resembled the powder used in the successful production of auxetic PP fibers and cylinders in terms of aspect ratio and morphology. From previous results [20], the size of powder particles was small enough to be processed in the extruder set up and was commensurate with the size distribution required to form an optimum network microstructure.

Measurement of Poisson’s Ratio

Figure 7a shows four cycles of length and width data obtained for a conventional (analogue) polyester fiber produced at 230°C with screw and take-up speeds at 0.525 rad/ second and 0.075 m/second, respectively. It was observed that the length and width data were out of phase by 180°, clearly showing that the width decreased as the length increased in response to the applied force along the length of the fiber. Similarly, the width increased as the length decreased as the applied tensile load is removed. This fiber, therefore behaved in a conventional (positive Poisson’s ratio) manner.

However, the fibers produced at 225°C with screw speed 0.525 rad/second and take-up 0.075 m/second were found to have in-phase length-width data (see Figure 7b). It can be observed that the width increased as the length increases in response to the applied force along the length of the fiber. Similarly, the width decreased as the length decreased on removal of the tensile load. Thus the fiber was confirmed to be auxetic.

Figure 8 shows the true lateral strain against the true axial strain from the third extension phase for both the auxetic and conventional polyester fibers. The auxetic polyester fibers, produced at 225°C with screw speed 0.525 rad/s and take-up 0.075 m/s, were found to possess Poisson’s ratio = -0.72 ± 0.05. However, the average Poisson’s ratio of the conventional fibers was found to vary between +0.18 and +0.25, which is typical for an engineering material.

Discussion

Herein the successful extension of the range of polymers that can be made in auxetic fiber form to include polyester is reported.

During the course of polyester extrusion, work was carried out with screw speed and take-up values that were previously employed for the successful production of auxetic PP fibers. However, the viscosity of the extruded fibers was too low to wind them with the take-up speed of 0.03 m/second while maintaining the same screw speed of 1.05 rad/ second. The main approach for the production of auxetic fibers lies in maintaining the minimum draw ratio. The minimum take-up speed maintained in the course of extrusion was found to have direct impact on screw speed. It was found that the optimum processing conditions for extrusion could be obtained by reducing the screw speed to 0.525 rad/second. Further reductions in the screw speed were found to be inappropriate to the production of auxetic polyester fibers. By extensive experimentation, it was found that the minimum take-up speed with which the extrusion of fibers would be successful was 0.075 m/second. Table 1 describes the processing conditions of 3GT powder to obtain the processing window for auxetic polyester fibers. It can be observed from the table that processing temperature plays a critical role in governing the Poisson’s ratio of polyester fibers. Although few traces of auxeticity have been detected in the fibers processed at 220°C, the auxeticity of the polyester fibers is significantly increased in the fibers processed at a processing temperature of 225°C.

Although auxctic and conventional (analogue) polyester fibers have shown to have a similar range of initial Young’s moduli, varying between 1.1 and 1.2 GPa, compared with the previous auxetic PP fiber that ranged between 1.34 and 1.5 GPa, the potential applications for auxetic polyester fibers are much greater in comparison with previous auxetic PP fibers. For example, auxetic fibers have potential in personal and sports protective clothing [21], such as bulletproof vests, helmets and shin pads; biomedical applications [22, 23], such as smart wound-healing bandages, ligaments and sutures; and as self-cleaning filters. These fibers could also play a significant role in the enhancement of fiber-reinforced composite toughness [24]; that is, more resistance to pull the fiber out from the composites. Currently, composites comprising conventional fiber and matrix material undergo lateral contraction in both the matrix and fibrous materials when loaded in tension axially, leading to failure at the fiber-matrix interface. On the other hand, a composite that employs auxetic fibers should allow the possibility of maintaining this interface to higher tensile loads by careful matching of the radial expansion of the auxetic fibers to the radial contraction of the conventional matrix.

However, in order to use auxetic fibers in these and other applications it will be necessary to produce auxetic fibers with other mechanical properties (e.g. strength and very high modulus) equivalent to the conventional fibers currently used in the application. This will require further work to increase the range of the materials that can be made in auxetic fiber form, and the development of predictive models to understand the deformation mechanisms leading to auxetic behavior in fibers.

Conclusions

The range of polymers that can be made in auxetic fiber form has been extended to include polyester. The fibers produced using thermal melt extrusion with a processing temperature of 225°C (flat profile across all the zones of the extruder), with screw speed 0.525 rad/second and takeup 0.075 m/second have been shown to be auxetic. Auxetic extruded products in the form of fibers have potential in a variety of applications, including fiber reinforcements in composite materials, protective clothing, fibrous filters and biomedical materials.

Acknowledgements

The authors would like to thank Dr Arun Pal Aneja (DuPont) for supplying polyester granules. Special thanks to Mrs Stella Peel (University of Bolton) for her assistance in the in-house grinding work on polyester granules.

2 Part of this paper was presented at Second International Conference on Materials for Advanced Technologies & IUMRS International Conference in Asia, Singapore, 2003.

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Naveen Ravirala, Kim L. Alderson, Philip J. Davies, Virginia R. Simkins and Andrew Alderson1

Centre for Materials Research & Innovation, University of Bolton, Deane Road, Bolton, BL3 5AB, UK

1 Corresponding author: Tel: +44 1204 903513; fax: +44 1204 903088; e-mail: a.alderson@bolton.ac.uk

Copyright Textile Research Institute Jul 2006

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