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Auxetic polypropylene films.

INTRODUCTION

Auxetic materials [1, 2] are those which when stretched expand rather than contract in width. In other words, they display negative Poisson's ratio behavior. A variety of products have been fabricated in auxetic form including polymeric and metallic foams [2-5], honeycombs [6, 7], polymer gels [8], and microporous polymers in the form of cylinders [9, 10] and fibers [11, 12]. Auxetic materials are predicted, and have been experimentally verified, to have enhanced properties. For example, the indentation resistance of auxetic materials such as microporous polymers [13], foams [5], and composites [14] were found to be enhanced by up to three times when compared with conventional materials. Other benefits of auxetic materials include enhancements in plane strain fracture toughness [15], ultrasonic energy absorption [16], and shear resistance [3, 17].

Auxetic behavior in microporous polymers was first observed in a particular form of polytetrafluoroethylene (PTFE) [18, 19]. Later, thermal routes were used to fabricate auxetic cylinders of ultrahigh molecular weight polyethylene (UHMWPE) [9, 13, 16], polypropylene (PP) [10], and nylon [20], consisting of three distinct stages: compaction, sintering, and ram extrusion. It was found that the most critical parameter governing the presence of auxeticity in the cylinders was the processing temperature. Other parameters such as die geometry, sintering time, structural integrity, and extrusion rates were also shown to have some effect on the auxeticity of the material. However, although useful for laboratory-based testing, the auxetic cylindrical rods were not ideal for applications-based research.

More recently, research has concentrated on the production of a more useful form of auxetic microporous polymer, namely auxetic fibers [11], which could lead to more application-based research. A novel thermal processing technique involving melt spinning in an extruder was employed to produce auxetic PP fibers. The fibers were extruded at 159[degrees]C with a screw speed 1.05 rad [s.sup.-1] and 0.03 m [s.sup.-1] take-up speed. These processing conditions have subsequently been found to be the optimum conditions for auxetic PP fibers in a detailed and systematic investigation into the variation of each of the key processing parameters (P.J. Davies, K.L. Alderson, A. Alderson, V.R. Simkins, and G. Smart, unpublished results). Characterization of the fibers involved optical techniques (video extensometry) to measure the strains across both the length and the width simultaneously during deformation of the fibers in a microtensile stage.

This article reports the successful production of auxetic PP films using the same method and same powder but with a different die geometry to that employed in the production of auxetic PP fibers. Characterization has been carried out by using both Instron tensile testing and Deben microtensile testing machines in combination with video extensometry.

EXPERIMENTAL

Characterization of PP Powder

The polymeric powder used for extrusion was commercially available Coathylene PB0580, supplied by Univar plc. This is the same powder used in the previous successful production of auxetic PP fibers and cylinders [10, 11].

[FIGURE 1 OMITTED]

In order to establish the thermal processing window, the thermal characteristics of the PP powder were established using differential scanning calorimetry (DSC). The sample was heated under flowing nitrogen at a heating rate of 10[degrees]C/min from ambient temperature (25[degrees]C) to 250[degrees]C.

A Cambridge S200 scanning electron microscope (SEM) was employed to characterize the PP powder in terms of particle size distribution, surface roughness, and shape. UTHSCSA Image-Tool-3 software [21] was employed to analyze the micrographs obtained from the SEM.

Extrusion of PP Powder

The extrusion was carried out using an Emerson and Renwick Ltd Labline extruder employing an Archimedean screw mechanism consisting of a 25.4 mm diameter screw, a 3:1 compression ratio, a length-to-diameter ratio of 24:1, five temperature zones each having individual thermostatic controls, thermocouples to measure the processing temperatures of the five zones of the extruder, a die slot with slit orifice (63.5 X 14.2 X 0.38 mm), and rollers to enable drawing and winding of the extruded products (Fig. 1). The PP powder was fed at a constant rate into the barrel through the feeder. To start with, the only change in the production of auxetic PP films compared to the production of auxetic PP fibers was the change in the die geometry from hole to slit orifice. As there was a change in the cross-section of die-head from circular (fibers) to rectangular (films), a detailed study was centered on the critical processing parameters leading from the die-head zone. A standard set of processing parameters was defined, based on those used previously [11] to produce auxetic PP fibers: flat temperature profile of 159[degrees]C, screw speed of 1.05 rad [s.sup.-1] and take-up speed of 0.03 m [s.sup.-1]. Each parameter was then varied in turn, while maintaining the other parameters at the standard values. Table 1 summarizes the experiments conducted to extrude the PP films. Conventional PP films were also extruded at 230[degrees]C using the same powder.

Characterization of PP Films

Initial mechanical properties characterization of the extruded PP films was carried out using a MESSPHYSIK ME 46 video extensometer [22] to measure the axial and transverse strains in test specimens undergoing uniaxial tension in an Instron tensile testing machine (Model No. 4303). Figure 2 shows schematically the video extensometer and tensile testing machine setup.

PP films were prepared in the form of dog-bone structures of thickness 0.15 mm, width 15 mm, and gauge length 40 mm. Markers were drawn on the samples to describe a box of dimensions 5 X 40mm as shown in Fig. 3. Tensile testing of the films was carried out at a 3 mm/min strain rate, with a 100 N load cell.

In order to check that possible out-of-plane deformation of the dog-bone samples was not leading to spurious apparent in-plane strains, smaller test specimens (5 X 10 mm) were tested in a Deben microtensile testing stage (Microtest) and again video extensometry was used to measure the strains. Each of these tests included mounting the sample between the jaws of the Deben microtensile testing stage with markers drawn ~4 mm apart in the axial and ~4.5 mm in the transverse direction. In the Deben microtensile stage tests, a load was cyclically applied and removed (up to four cycles) to a maximum strain of 1% at an extension rate of 0.1 mm/min, with a 75 N load cell.

The camera of the video extensometer captured the image of the PP film. The video extensometer software operated directly as a strain meter by determining the relative change in distance between the fiducial marks along the length and width by tracking the change in the contrast between the markers and sample as strain is applied to the specimen. The video extensometer personal computer (PC) thus records axial and transverse length data during the tensile test and therefore enables the axial and transverse strains to be calculated. Transverse width data is collected for 10 sections along the length of the film, enabling the individual width section data to be generated as well as an average width data set. The PC attached to the tensile testing unit gave the load-extension data for each test.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Tests were performed for uniaxial loading of the films both along and transverse to the direction of extrusion (x and y directions, respectively).

RESULTS

Differential Scanning Calorimetry

The DSC curve for the PP powder (Fig. 4) displays the characteristics previously observed for this powder [23]: melt onset temperature (the temperature at which the polymer just starts melting) of 130[degrees]C and peak melting temperature of 161[degrees]C. Since the process to produce auxetic melt extruded products relies on only partial melting of the polymeric powder, then the DSC curve confirms the flat temperature profile in the region of 159[degrees]C chosen for this work (based on the previous production of auxetic PP fibers [11]) is reasonable in the attempt to produce auxetic PP films.

[FIGURE 4 OMITTED]

Scanning Electron Microscopy

Analysis of Fig. 5 shows the PP powder particles to have a rough morphology and a distribution of particle sizes. The average particle aspect ratio was measured to be 1.87 and the particles have a dimension of the order of 50 [micro]m. This is again consistent with previous findings for this polymer powder and has the suggested necessary features to be processible in an auxetic extruded form [10].

[FIGURE 5 OMITTED]

Characterization of the Mechanical Properties of PP Films Using the Instron Tensile Testing Machine and Video Extensometer

Figure 6a shows the width versus length data for PP films obtained by video extensometry during tensile testing of the films loaded along the direction of extrusion in the Instron tensile testing machine. The PP films were produced with a 159[degrees]C processing temperature and screw speed of 1.05rad [s.sup.-1]. Take-up speeds were varied between 0.015 to 0.0375 m [s.sup.-1]. It can be seen that the PP film processed with a take-up speed of 0.0225 m [s.sup.-1] (Fig. 6a; ii) shows an initial increase in width with an increase in the length, in response to a tensile load. When the length has increased to ~39.6 mm, the width then starts to decrease with further increases in length. Thus the film is auxetic at low strain becoming conventional at higher strain (~1%). The PP films produced at the lower and higher take-up speeds (Fig. 6a; i, iii, and iv) show conventional (positive Poisson's ratio) behavior throughout the deformation, i.e., the width continuously decreases with increase in length. Figure 6b shows the corresponding load-extension curves. The data were observed to be reproducible in subsequent tests.

Mechanical Properties Characterization of PP Films Using the Deben Microtensile Testing Stage and Video Extensometer

Figure 7a shows the four cycles of length and average width data obtained for loading along the direction of extrusion of the PP film produced at 230[degrees]C with screw and take-up speeds of 1.05 rad [s.sup.-1] and 0.0225 m [s.sup.-1], respectively. The length and width data are out of phase with a phase difference of 180[degrees], i.e., the width is decreasing as the length increases in response to the applied tensile force along the length of the film. This is, therefore, the characteristic behavior for a conventional positive Poisson's ratio film.

[FIGURE 6 OMITTED]

The PP films extruded at 159[degrees]C with screw speed 1.05 rad [s.sup.-1] and take-up speed 0.0225 m [s.sup.-1], on the other hand, show the length and width to be in phase for uniaxial loading both along and transverse to the direction of extrusion (Fig. 7b and c, respectively). Figure 7b shows the width increases as the length increases in response to the applied tensile force along the length of the film. Figure 7c shows the length increases as the width increases in response to the applied tensile load along the width of the film (transverse to the direction of extrusion). Thus this particular PP film is confirmed to be auxetic in both principal in-plane directions.

Measurement of Poisson's Ratio

From Fig. 7 it is clear that the length and width variations are repeatable for the load cycles employed in the tests. Hence, data from the extension phases of the second, third, and fourth cycles were analyzed and averaged for the evaluation of the value of the Poisson's ratio for each combination of film and loading direction tested. The true axial and lateral strains were calculated from the length-width data obtained from the video extensometer. The true axial strain ([[epsilon].sub.x]) and true lateral strain ([[epsilon].sub.y]) are given by

[[epsilon].sub.x] = ln(l/[l.sub.0]) (1)

[[epsilon].sub.y] = ln(w/[w.sub.0]) (2)

where, [l.sub.0] and [w.sub.0] are, respectively, the original length and width of the film, and l and w are the deformed length and width of the film, respectively.

The Poisson's ratio [[nu].sub.xy] of a material under tension (or compression) in the x direction is defined by

[[nu].sub.xy] = -[d[[epsilon].sub.y]]/[d[[epsilon].sub.x]] (3)

which is applicable to both linear and nonlinear elastic deformation [24, 25]. The in-plane Poisson's ratio is, thus, calculated from the negative of the slope of the lateral strain versus axial strain plot. Interchanging the subscripts x and y in Eq. 3 yields the Poisson's ratio ([[nu].sub.yx]) for uniaxial loading in the y direction. In this article, the Poisson's ratio is only calculated at low strain (<0.1%) to confirm the presence or otherwise of auxetic behavior. A least squares best-fit straight line was used in all cases.

Figure 8 shows the [[epsilon].sub.y] versus [[epsilon of].sub.x] data for uniaxial loading along the extrusion (x) direction of the PP films extruded at 230[degrees]C with screw speed 1.05 rad [s.sup.-1] and take-up speed of 0.0225 m [s.sup.-1]. The slope of the least squares best-fit straight line to the data in Fig. 8 yields an in-plane Poisson's ratio for this film of [[nu].sub.xy] = +0.38. This lies within the range of Poisson's ratio values for most typical materials (+0.2 [less than or equal to] [nu] [less than or equal to] +0.4).

Figure 9a and b shows the [[epsilon].sub.y] versus [[epsilon].sub.x] data for the auxetic films (extruded at 159[degrees]C, with screw speed 1.05 rad [s.sup.-1] and take-up speed 0.0225 m[s.sup.-1]) loaded along the direction of extrusion, characterized using the Instron tensile testing machine and Deben microtensile testing machine (third extension phase), respectively. In Fig. 9a, the best fit straight line is offset from the origin (0,0), indicating there is scatter of the data points in the vicinity of the origin. However, since it is the slope that determines the sign and magnitude of the Poisson's ratio, then the presence or otherwise of an offset to the best-fit line does not affect the value of Poisson's ratio thus determined. Figure 9a yields an in-plane Poisson's ratio for loading along the direction of extrusion, derived from the Instron tensile testing machine data, of [[nu].sub.xy] = -0.75 [+ or -] 0.05.

[FIGURE 7 OMITTED]

The data for axial loading in the Deben microtensile testing machine shown in Fig. 9b indicate a strain dependency for the Poisson's ratio not evident in the Instron data for the strain range covered in Fig. 9. Nevertheless, a straight line with the slope equal to that obtained from the Instron data shows a good fit to the data over the major intermediate strain range (0.0001 < [[epsilon].sub.x] < 0.0019; see dashed line in Fig. 9b).

[FIGURE 8 OMITTED]

The low strain ([[epsilon].sub.x] < 0.1%) value of the in-plane Poisson's ratio for the auxetic film undergoing loading along the film extrusion direction is calculated from the Deben microtensile testing machine to be [[nu].sub.xy] = -1.15 for the third cycle extension phase (Fig. 9b; solid line). Averaging over the extension phases for the second, third, and fourth cycles yields a value of [[nu].sub.xy] = -1.12 [+ or -] 0.06.

Figure 9c is a plot of the [[epsilon].sub.x] versus [[epsilon].sub.y] data for the auxetic film undergoing loading perpendicular to the direction of extrusion in the Deben microtensile testing machine (third extension phase). A value for the Poisson's ratio for loading perpendicular to the extrusion direction of [[nu].sub.yx] = -0.78 is derived from the best-fit straight line to the data for [[epsilon].sub.y] < 0.001. Averaging the calculated values for the second, third, and fourth cycles yields [v.sub.yx] = -0.77 [+ or -] 0.01.

Figure 10 shows the effect of processing temperature on the Poisson's ratio ([[nu].sub.xy]) of the PP films. Data points are shown for the Poisson's ratios calculated from the least squares best-fit straight lines to the strain-strain data for loading strains in the range (0 < [[epsilon].sub.x] < 0.003) for each of the 10 individual width sections generated by the video extensometer during each test. It can be seen that the processing temperature window for auxetic behavior is only 1-2[degrees]C around the established temperature of 159[degrees]C.

Figure 11 shows the effect of variation in the screw speed on the PP film Poisson's ratio [[nu].sub.xy] (calculated for 0 < [[epsilon].sub.x] < 0.003). The choice of a screw speed of 1.05 rad [s.sup.-1] as the optimum speed is confirmed for the range and increments of speed used in this investigation. There is some evidence of auxetic behavior at the lower screw speed of 0.525 rad [s.sup.-1], but all auxetic character is lost at the higher screw speed of 1.575 rad [s.sup.-1].

The effect of take-up speed on Poisson's ratio [[nu].sub.xy] (calculated for 0 < [[epsilon].sub.x] < 0.003) is plotted in Fig. 12. The take-up speed of 0.0225 m [s.sup.-1] was confirmed as the optimum take-up speed, with auxetic character being lost for all other take-up speeds employed.

Measurement of Young's Modulus

The Young's moduli can be calculated from the slope of the stress-strain curves derived from the raw load-elongation data in, for example, Fig. 6. Figure 13 shows the stress-strain data for uniaxial loading along the extrusion direction of the film processed at 230[degrees]C (Fig. 13a), uniaxial loading along the extrusion direction of the film processed at 159[degrees]C (Fig. 13b), and uniaxial loading perpendicular to the extrusion direction of the film processed at 159[degrees]C (Fig. 13c). The data in Fig. 13 correspond to films processed at a screw speed of 1.05 rad [s.sup.-1] and take-up speed of 0.0225 m [s.sup.-1] in all cases.

The calculated Young's modulus along the extrusion direction of the conventional film was found to be [E.sub.x] = 0.61 [+ or -] 0.03 GPa, which compares with a value of [E.sub.x] = 0.34 [+ or -] 0.01 GPa for the axial Young's modulus of the auxetic film. The transverse Young's modulus of the auxetic film was found to be [E.sub.y] = 0.20 [+ or -] 0.01 GPa.

Table 2 summarizes the experimentally measured mechanical properties (Poisson's ratios and Young's moduli) for the conventional and auxetic PP films.

DISCUSSION

It can be seen that each of the load-extension curves in Fig. 6b show no evidence of slippage while loading. Also there is no initial flat curve for the data, the presence of which could have indicated a slack test sample at the beginning of the test. An initially slack test sample could lead to an apparent negative Poisson's ratio as the film flattens into the plane when the slack is taken up during the test. Hence the load-extension data from the Instron tensile testing machine indicate the negative Poisson's ratio measured using the video extensometer (Fig. 6a; ii) is genuine. This is confirmed by the Deben microtensile stage/video extensometer data in Fig. 7b.

The detailed investigation into the processing parameters for auxetic PP films reported in this article is similar to that performed in the study of the processing window for auxetic PP fibers (P.J. Davies, K.L. Alderson, A. Alderson, V.R. Simkins, and G. Smart, unpublished results).

The processing temperature window for auxetic behavior in PP films has been found to be exceedingly narrow, corresponding to only 1-2[degrees]C at most around the optimum temperature of 159[degrees]C. However, this is consistent with previous work into the production of auxetic PP cylinders [10] and fibers [11]. From previous thermal analysis using differential scanning calorimetry it is thought that the processing temperature is related to the onset of melting of the powder particles [23]. At this characteristic temperature the powder particles undergo surface melting, which it is thought enables the formation of a network microstructure (which is also aided by the more efficient packing of particles possible when a size distribution (Fig. 5) is present in the powder). With the appropriate choice of other processing parameters the network microstructure can then be formed into a geometry leading to auxetic behavior (see, for example, Refs. 26 and 27). Processing at a temperature below the characteristic temperature does not lead to surface melting, while processing above the characteristic temperature leads to full melting of the powder particles. In both these latter cases the formation of a network microstructure capable of leading to auxetic behavior is not possible. For PP the onset of melting is particularly sharp with respect to temperature (Fig. 4), leading to the narrow processing temperature window for auxetic behavior in this polymer. Conversely, the onset of melting in UHMWPE is more gradual with temperature, leading to a larger processing window for auxetic UHMWPE cylinders [28].

[FIGURE 9 OMITTED]

The optimum screw speed required for processing auxetic PP films has been found to be 1.05 rad [s.sup.-1]. The effect of screw speed on the processing of auxetic fibers is likely to be complex. On the one hand, the screw speed clearly influences the time the polymer experiences at the processing temperature, and so an optimum value will be expected to correspond to the time required for surface melting to occur. Any faster than this optimum value of screw speed will lead to insufficient surface melting of the powder particles and subsequent hindering of network microstructure formation. However, increasing the screw speed will also lead to shear heating of the polymer, which will raise the temperature experienced by the polymer, tending therefore to counteract the effect of the reduced residence time. A slower screw speed than the optimum value will lead to eventual full melting of the powder particles, again mitigating against the formation of an appropriate network microstructure. This will be counteracted to some degree by the reduced contribution of shear heating to the temperature experienced by the polymer. The screw speed will also have an effect on the compaction of the powder particles as they traverse the screw. Compaction conditions have also been found to be important in the production of auxetic extruded products, in particular on the structural integrity of the product [29].

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

[FIGURE 13 OMITTED]

Varying the take-up speed has also been found to have an effect on the Poisson's ratio of the PP films (Fig. 12). The optimum take-up speed for processing auxetic films was found to be 0.0225 m [s.sup.-1]. Processing with a lower value than the optimum value is seen to lead to a loss of auxetic character, indicating that a small amount of drawing of the film is required for the network microstructure to adopt the geometry required for auxetic behavior. However, applying too much drawing of the film, by increasing the take-up speed to above the optimum value, leads to a loss of auxetic character. This is consistent with the network microstructure being drawn into a more open geometry leading to positive Poisson's ratio behavior. It is interesting to note that the optimum take-up speed was found to be slightly lower for the production of auxetic PP films (0.0225 m [s.sup.-1]) than that required for auxetic PP fibers (0.03 m [s.sup.-1]) [11].

The auxetic PP film data from the Instron tensile testing machine show a strain dependency in the Poisson's ratio behavior (Fig. 6a; ii). The film was found to be auxetic up to a strain of ~1%, whereupon an auxetic-to-nonauxetic (negative-to-positive Poisson's ratio) transition occurred for further tensile deformation. The film possesses a positive Poisson's ratio of [[nu].sub.xy] [approximately] +0.4 at strains > 1%. Characterization using the Deben microtensile stage confirmed this strain dependency and auxetic-to-nonauxetic transition (Fig. 14). Figure 14 includes the width data for each section along the length of the test specimen, as well as the average width curve, demonstrating the uniformity of the Poisson's ratio behavior throughout the length of the specimen. The auxetic-to-nonauxetic transition occurs at ~0.4% strain in the Deben microtensile stage test. The discrepancy in the transition strain between the Instron tensile testing machine and Deben microtensile stage tests is possibly attributable to slight variations between the test specimens, differences in the test specimen geometries, and the different strain rates employed in the two tests. A uniaxial stress induced auxetic-to-nonauxetic transition has previously been observed in auxetic PTFE [18, 26, 27] and UHMWPE cylinders [24]. Furthermore, the strain dependency in PTFE and UHMWPE has also been predicted from microstructural models [26, 27].

[FIGURE 14 OMITTED]

The table summarizing the mechanical properties for the films (Table 2) indicates a slight anisotropy in the in-plane properties of the auxetic film. This may be attributable to the small amount of drawing of the film found to be necessary to impart auxetic character to the film. For an anisotropic film obeying classical elasticity theory, thermodynamic considerations require the material to have a symmetric compliance matrix [30], i.e.,

[FIGURE 15 OMITTED]

[[nu].sub.xy][E.sub.y] = [[nu].sub.yx][E.sub.x]. (4)

From Table 2, [[nu].sub.xy][E.sub.y] = -0.22 [+ or -] 0.02 GPa and [[nu].sub.yx][E.sub.x] = -0.26 [+ or -] 0.01 GPa, indicating that the measured properties for the auxetic PP film are in reasonable agreement with classical elasticity theory.

The axial Young's modulus for the conventional film ([E.sub.x] = 0.61 GPa) is seen to be higher than that for the auxetic film ([E.sub.x] = 0.34 GPa). The conventional film is processed at 230[degrees]C and so the polymer undergoes full melting during the extrusion process. Hence the mechanism responsible for the Young's modulus in the conventional film is likely to be due to molecular chain effects. For the auxetic film, on the other hand, the processing temperature of 159[degrees]C results in only partial (surface) melting of the polymer particles, and so the mechanism responsible for the Young's modulus is likely to be due to the formation of an interconnected network of polymer particles at the microstructural level. A detailed explanation of the axial Young's moduli for the conventional and auxetic films will, therefore, require a study of the structure-deformation relationships at the molecular and microstructural scales, respectively.

The value of [E.sub.x] = 0.61 GPa for the conventional film is significantly (one to two orders of magnitude) lower than that usually associated with PP films. However, it is usual for PP films to undergo significant drawing during processing to achieve high Young's moduli values due to molecular chain alignment. In this article, the conventional film was produced with only a relatively low amount of drawing for comparison with the auxetic film (in which it is essential to avoid significant drawing). It is expected this will lead to little molecular chain alignment in the conventional film reported here and, therefore, a reduced Young's modulus compared to commercial PP films.

The high volume change and concomitant enhanced permeability variation with strain for porous auxetic materials has been shown to lead to cleanable and selective/tunable filter materials [31], leading to potential for auxetic films in separations, breathable membrane and drug delivery applications. We expect the enhanced plane strain fracture toughness due to the presence of a negative Poisson's ratio, together with the self-healing nature of an auxetic material containing a cut or tear while undergoing a tensile load, should lead to auxetic films and membranes displaying enhanced tear resistance (Fig. 15).

CONCLUSIONS

The range of auxetic materials has now been extended to include PP films, based on the thermal processing route developed for the production of auxetic PP fibers. The auxetic PP films produced at 159[degrees]C with screw speed 1.05rad [s.sup.-1] and 0.0225 m [s.sup.-1] were found to have low strain (<0.1%) in-plane Poisson's ratios of [[nu].sub.xy] = -1.12 and [[nu].sub.yx] = -0.77, and Young's Moduli of [E.sub.x] = 0.34 GPa and [E.sub.y] = 0.20 GPa. The auxetic effect persists for tensile strains up to ~1%. At a tensile strain of ~1% the auxetic film undergoes an auxetic-to-nonauxetic transition. Systematic variations of each of the key processing parameters have been performed to define the processing window for auxetic behavior. The effect of each of the processing parameters on the ability to produce an auxetic film has been discussed.
TABLE 1. Extrusion matrix for PP films.

Temperature [degrees]C Take-up (m
(at all the zones) Screw speed (rad [s.sup.-1]) [s.sup.-1])

159 0.525 0.03
 1.05
 1.575
 1.05 0
 0.015
 0.0225
 0.0375
 0.045
 0.06
158 1.05 0.015
 0.0225
 0.03
 0.0375
160 1.05 0.015
 0.0225
 0.03
 0.0375
161 1.05 0.015
 0.0225
 0.03
 0.0375
162 1.05 0.015
 0.0225
 0.03
 0.0375
230 1.05 0.0225

TABLE 2. Experimentally determined low strain (<0.1%) mechanical
properties of PP films processed with a screw speed of 1.05 rad
[s.sup.-1] and take-up speed of 0.0225 m [s.sup.-1].

Processing temperature
([degrees]C) [v.sub.xy] [v.sub.yx]

159 -1.12 [+ or -] 0.06 -0.77 [+ or -] 0.01
230 +0.38 [+ or -] 0.02 --

Processing temperature
([degrees]C) [E.sub.x] (GPa) [E.sub.y] (GPa)

159 0.34 [+ or -] 0.01 0.20 [+ or -] 0.01
230 0.61 [+ or -] 0.03 --


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N. Ravirala, A. Alderson, K.L. Alderson, P.J. Davies

Centre for Materials Research and Innovation, The University of Bolton, Deane Road, Bolton BL3 5AB, United Kingdom

Correspondence to: A. Alderson; e-mail: a.alderson@bolton.ac.uk
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Author:Ravirala, N.; Alderson, A.; Alderson, K.L.; Davies, P.J.
Publication:Polymer Engineering and Science
Date:Apr 1, 2005
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