The effects of powder morphology on the processing of auxetic polypropylene (PP of negative Poisson's ratio).
Since 1987, synthetic materials that possess a negative Poisson's ratio (v) have been fabricated. The first material to show this effect was a novel foam (1), which expanded both transversely to and along the direction of an applied load. Subsequently, the range of synthetic materials capable of existing in auxetic forms has included polymer gels (2), molecular structures (3), and microporous polymers (4-6).
The first microporous polymer to be identified was a particular form of polytetrafluoroethylene (PTFE), which was found to be auxetic solely because of its complex microstructure. This consists of an interconnected network of nodules and fibrils that react cooperatively to produce a negative Poisson's ratio, indicating that if this microstructure could be reproduced in other polymeric materials then so might the negative Poisson's ratio effect.
In 1992, the nodule-fibril microstructure, which leads directly to auxetic behavior, was reproduced in ultra high molecular weight polyethylene (UHMWPE) by means of a novel thermal processing route (7). This consisted of three distinct stages - compaction of finely divided polymer powder (8), sintering (9), and extrusion through a die (10). The extrudates produced possessed a strain dependent negative Poisson's ratio in compression (11, 12) which, depending on the processing parameters utilized, approached v = -6 at low strains for low modulus extrudates and v = -1.5 again at low strains (typically 2-3%) for extrudates possessing a modulus of 0.2 GPa (13). This is a high enough modulus for the auxetic UHMWPE to be considered as a structural material, and, in combination with the theoretical advantages (1, 14, 15) of enhanced mechanical properties such as improved shear modulus, indentation resistance, and plane strain fracture toughness for auxetic materials, represents a potentially useful, novel material. Indeed, recent work has shown that at low loads, indentation resistance is considerably enhanced (i.e. more than a factor of 3) for the structural form of auxetic UHMWPE when compared with conventionally processed forms (16).
Since both PTFE and UHMWPE exhibit auxetic behavior solely because of their microstructures and not because of any intrinsic material property, it should be possible to develop processing routes that would produce similar behavior in other polymers. This paper examines one such common thermoplastic, namely polypropylene (PP), in which particular emphasis is placed, for the first time, on the importance of powder morphology on the ability of PP to produce an auxetic material via processing using a thermal route based on that successfully employed for UHMWPE.
Microscopic Examination of the Powder Morphologies of Three Grades of Polypropylene Powder
As one of the major areas of this work was to examine the effects of powder morphology on the ability of a polymer to be processed into an auxetic form, the initial area of experimental work concerned examination of the powder morphology of three grades of PP powder, to be referred to as PP1, PP2, and PP3. PP1 is a commercially available PP grade supplied by ICI (17). PP2 is again supplied by ICI, but is not a commercially available grade, rather it was specifically requested for this project. The conventional production of PP powder is aimed at producing smooth, regular particles that flow easily for conventional polymer processing. To obtain a rougher particle surface and a particle size distribution for comparison purposes, ICI supplied unstabilized PP removed from the production plant in the early stages of production. PP3 is a further, commercially available, PP grade, supplied by K and K Greeff (18).
In all cases, samples of each powder were mounted on aluminum stubs and gold coated using a sputter coater to establish a well formed conducting path. The powder morphologies were then observed at magnifications of up to x640 using a Phillips 501 SEM.
Thermal Processing Route for the Production of Auxetic Polymers
For all grades of PP powder under consideration, the production of auxetic PP was based on the thermal method successfully employed previously (6-10) to produce auxetic UHMWPE. For completeness, this is summarized below.
The entire process was performed using the rig shown in Fig. 1. This consists basically of a barrel of bore diameter either 10 mm or 15 mm that is heated via an external band heater. Interchangeable dies are located in the bore of the barrel, including a blank die to allow compaction of the powder to take place. The actual processing route can be divided into three distinct stages: compaction of polymer powder, sintering, and extrusion. The pressure required for the compaction and extrusion stages is applied via a brass tipped ram that is attached to a Schenk-Trebel electromechanical testing machine. With the barrel heated to 110 [degrees] C and the blank die in position, polymer powder was poured into the barrel. This was then allowed to come to equilibrium for 10 min (referred to as the stand time), after which the ram was lowered into the bore of the barrel at a rate of 20 mm/min until a pressure of 40 MPa was reached. This pressure was maintained for 20 min producing a contiguous rod of material, which was allowed to cool and then removed from the barrel.
The compacted rod was then sintered at 160 [degrees] C for 20 min and then extruded, at 500 mm/min, through a conical die of entry diameter 15 mm (if a 15 mm bore barrel was used), exit diameter between 7 and 7.5 mm, cone semi-angle 30 [degrees] , and capillary length 3.4 mm. This die geometry was selected, from previous work, to produce high modulus, auxetic material (10).
In this investigation of the processing of the three grades of PP, these processing variables were varied and the effect on the extrudates produced was measured.
Measurement of the Poisson's Ratio of the Extrudates
The processed extrudates were produced as cylinders of up to 11.5 mm in diameter and 90 mm in length. From these, short cylinders 4 mm thick were cut. The radial Poisson's ratio, [v.sub.rz], was measured by compressing the samples in the radial direction, r, and measuring the change in thickness in the axial direction, z. The change in dimensions was recorded photographically using a Wild M8 stereo microscope from which the Poisson's ratio
[v.sub.rz] = - [[Epsilon].sub.z]/[[Epsilon].sub.r] (1)
at a particular strain, [[Epsilon].sub.r], could be calculated. By using different degrees of compressive strain, it was possible to obtain the full strain history of the samples using this very simple but accurate technique.
The results obtained using each of the three grades of PP under investigation are considered separately below.
Powder Morphology and Attempts at the Fabrication of PP1 to Produce Structural Auxetic Material
Figure 2 shows the powder morphology of PP1. It can be seen that PP1 consists of smooth, regular, spherical particles with an average size of [approximately]250 [[micro]meter]. This is in sharp contrast with the morphology of GUR 415 UHMWPE powder, supplied by Hoechst (19), which is the raw material for the production of auxetic UHMWPE. As can be seen from Fig. 3, GUR 415 UHMWPE powder consists of rough, irregular particles, with an average size of approximately 100 [[micro]meter], i.e. much smaller than that of PP1. It was recognized that these major differences in powder morphology may have adverse effects on the compactability and subsequent processing of PP1.
As stated earlier, the initial stage of the processing route was compaction of the polymer powder to produce rods that could be subsequently handled in the sintering and extrusion phases. Previous work (8) has shown that of the five processing variables involved in the compaction stage (i.e. compaction temperature, compaction rate, compaction load or pressure, stand time, and loading time), by far the most significant are temperature and pressure, with temperature showing the most effect. Therefore, using all variables as previously employed (8-10) for UHMWPE, PP1 was compacted at a range of temperatures from 110 [degrees] to 170 [degrees] C. Observation of the compacted rods by SEM revealed that at temperatures approaching the melting point of PP1 (i.e. 169 [degrees] C), the particles were subject to large scale deformation, resulting in their forming large, flat regions on the particle surfaces. As a result of this, the compacted rod produced is virtually free from voids, with very good densification observed. As the temperature was lowered, the particles retained their spherical shape, resulting in large voids between the individual particles until at temperatures [less than]120 [degrees] C, the particles did not compact at all and the rod disintegrated upon handling.
Compaction at 130 [degrees] C produced some evidence of interparticle fibrillation [ILLUSTRATION FOR FIGURE 4 OMITTED]. This was encouraging as it suggested the potential for producing auxetic behavior existed for PP1. Hence, a number of rods were compacted at 130 [degrees] C, keeping all other conditions as standard, and using a bore of diameter 10 mm. A die of entry diameter 10 mm, exit diameter 5 mm, capillary length 2 mm, and cone semi-angle 60 [degrees] was selected and fitted into the bore of the barrel. In all cases, a sintering time of 20 min and an extrusion rate of 500 mm/min were employed. The temperature at which sintering and extrusion were carried out was varied from 160 [degrees] C (the standard temperature used for the sintering and extrusion of UHMWPE to produce auxetic material) to 170 [degrees] C. At 160 [degrees] C, the extrudate that was produced broke into fragments upon emerging from the barrel ranging from 3 mm to 15 mm in length and [approximately]5 mm in diameter. In an attempt to improve the structural integrity of the extrudate so that compression testing could be carried out to measure the Poisson's ratio of the material, the sintering and extrusion temperature was increased to 165 [degrees] C. The extrudates thus produced again fragmented upon emerging from the barrel, with the only difference from that obtained at 160 [degrees] C being that the diameter of the extrudate had increased slightly. Larger but still fragmented samples were obtained by further increasing the temperature to 167 [degrees] C, these being of dimensions in the region of 7 mm in diameter and 20 mm in length. At 168 [degrees] C, an irregular but intact extrudate was produced, though this lacked any large degree of structural integrity. However, increasing the sintering and extrusion temperature by just 1 [degree] C, i.e. to 169 [degrees] C, did not produce the desired structural but auxetic PP; rather, PP1 under these conditions melted.
The poor results detailed here for the processing via a thermal route of PP1 indicated that either this particular form of PP had a powder morphology unsuitable for this type of processing route for auxetic polymers or that PP itself was unsuitable for the production of an auxetic polymer. Assuming the former to be the case, further investigations of PP2 and PP3 were undertaken.
Powder Morphology and Attempts at the Fabrication of PP2 to Produce Structural Auxetic Material
Figure 5 shows the powder morphology of PP2. As stated previously, this is not a commercially available PP grade, but had been specially produced by ICI for this work to have a rougher surface than their conventional PP grades. However, in producing a rougher surface as requested, the particles of PP2 are much larger than those of PP1, ranging from about 0.5 mm to 3 mm and appear to be aggregates of smaller particles. Despite the large, almost granular, nature of PP2, it was hoped that the roughness of the surface would be influential in the production of the nodule-fibril microstructure required for auxetic behavior.
Investigations into the processing of PP2 via the thermal route to produce an auxetic polymer commenced with the same compaction conditions as were employed for PP1 i.e. all conditions as defined as standard for UHMWPE except a compaction temperature of 130 [degrees] C. The result of this was that no degree of compaction of PP2 could be obtained. The compaction temperature was then increased further up to 160 [degrees] C along with increases in the second most significant of the processing variables, i.e. compaction pressure, up to 0.08 GPa, but compaction to produce a well formed rod for further processing was not obtained.
Despite the particle surface roughness, this particular grade of PP had granules that were too large for adequate compaction. To improve this, the granules of PP2 were mechanically ground to produce a smaller particle size. This had the desired effect of reducing particle size, but the intra-particle fracture surfaces produced were relatively smooth and flat, thus reducing the overall roughness of the particles. However, producing smaller particle sizes did enable a number of compacted rods with the required degree of structural integrity to be produced if a temperature of 160 [degrees] C and a pressure of 0.05 GPa were employed. These were subsequently sintered and extruded, but this again proved unsuccessful. The sintering and extrusion temperature was varied up to 171 [degrees] C. At 165 [degrees] C, the extrudates possessed no structural integrity (i.e. crumbled on handling) and had not expanded radially after exiting the die. This set of results was repeated at temperatures of 167 [degrees] C and 169 [degrees] C. Further increases in temperature resulted in the powder melting and flowing freely through the die without the application of an external force.
Once again, the poor results from this investigation of PP2 indicate that either PP2 is unsuitable for processing via a thermal route to produce an auxetic polymer or that PP itself is unsuitable. Again, the former was believed to be the case, and investigations into the third available grade of PP, PP3, were commenced.
Powder Morphology and Attempts at the Fabrication of PP3 to Produce Structural Auxetic Material
As a result of the earlier work a PP powder grade comprising small, rough particles was obtained (18) and Fig. 6 shows its powder morphology. It can be seen that PP3 has, of the three grades of PP investigated, the smallest particle size, this being in the region of 50 [[micro]meter]. The particles also have a rough surface and, by comparison with Fig. 3, most closely resembled the powder used successfully to produce auxetic UHMWPE. This, it was hoped, would then lend itself to successful processing via the thermal route of auxetic PP if, as seems likely, powder morphology has an effect on the ability of a polymer to form auxetic material.
PP3 was subsequently found to compact extremely well. Even at room temperatures, a compacted rod could be produced which, although very brittle and friable, remained in one piece. Increasing the temperature to 80 [degrees] C allowed the pressure used to compact PP3 to be lowered from 0.04 GPa to 0.02 GPa with no adverse effect on the quality of the compact. This represented a considerable improvement on the compactability of PP1 and PP2.
Although compaction of PP3 proved relatively straight forward, the sintering and subsequent extrusion stages of the processing route proved difficult, particularly on attempting to produce structural auxetic material. This investigation underlines work performed previously on producing structural auxetic UHMWPE, which emphasized the importance of die geometry in the extrusion stage (10). Initial work was carried out using a barrel with a bore of diameter 10 mm and a die of exit diameter 4 mm. Using this in conjunction with sintering and extrusion in the range of 145 [degrees] to 160 [degrees] C produced extrudates that were always unstable. At 160 [degrees] C, melting of the polymer occurred. Samples of the extrudates, though unstable, were examined in the SEM and for those processed at 155 [degrees] C, some evidence of fibrillation was observed, indicating that production of an auxetic material was possible.
In accord with the previous work on the extrusion parameters required to produce structural auxetic polyethylene (10), the exit diameter of the die was increased to 6 mm. Using this larger exit diameter die, continuous extruded rods were produced throughout the entire range of sintering and extrusion temperatures employed i.e. 130 [degrees] to 160 [degrees] C. The extrudates were, however, slightly different as the temperature increased in that radial expansion after extrusion increased as the temperature increased. For example, if a processing temperature of 130 [degrees] C was used, the rods expanded after extrusion to a radial diameter of 6.7 mm, whereas, if 159 [degrees] C was employed, the rods thus produced expanded radially to a diameter of 9 mm. However, because of the small size of the samples produced from the 10 mm diameter bore barrel, accurate measurement of any mechanical parameters (and in particular the Poisson's ratio) was difficult. Thus, the process was scaled up to produce larger samples in accordance with previous work to produce larger samples of UHMWPE (8-10), i.e. the ratio between the bore diameter of the barrel and the exit diameter of the die was maintained. This resulted in a die of exit diameter 9 mm being employed in conjunction with standard conditions, except that a sintering and extrusion temperature of 159 [degrees] C was used to prevent the polymer melting. The resulting extrudate was a rod of uniform cross section, with a diameter of 11.5 mm.
Measurement of the Poisson's Ratio of the Extrudates Produced from PP3
Compression tests were conducted on the extrudates produced, as described above, from PP3, and the Poisson's ratio was measured for several sections from six different samples. The results are presented in Fig. 7 and it can be seen that in all cases, v was not the conventional value expected for PP, which would be v = + 0.2. Indeed, for samples 1, 3, 5, and 6, all the values measured for v were negative and values up to -0.22 [+ or -] 0.01 at a strain of 0.016 were obtained. It appears that there is a great deal of scatter in these results. This arises from two sources. First, as with previous work on UHMWPE (10), the final microstructure is very sensitive to small variations in processing conditions and this will account for the scatter from sample to sample. However, for any one sample, v varies systematically with strain, and can be interpreted using a simple geometric model for the microstructure, described below.
Effect of the Powder Morphologies of PP1, PP2, and PP3 on their Ability to be Processed to Produce an Auxetic Polymer
This investigation has clearly shown that powder morphology is a very important parameter in the ability of a polymer to be produced in an auxetic form. Both PP1 and PP2 were found to be unsuitable. Considering PP1, the particles were thought to be unsuitable due to their round and smooth surfaces resulting in only few but relatively large contact points, which is not conducive to fibril formation but rather to conventional polymer processing, which is what this grade of PP is intended for. If a rough surface alone was necessary for the formation of auxetic material, it would be expected that PP2, which did indeed have a rough particle surface [ILLUSTRATION FOR FIGURE 5 OMITTED], would be capable of producing an auxetic end-product after processing via the thermal route. This proved to not be true, however, as the material was not amenable to compaction, a necessary prerequisite to the sintering and extrusion stages.
Thus, it can be concluded that to produce an auxetic polymer via this thermal processing route, the starting point should be a finely divided powder with a particle size up to about 300 [[micro]meter] and with a rough surface. These conditions were satisfied both for the UHMWPE thus far processed, to produce a structural auxetic polymer and for PP3, which also achieved this aim. The Poisson's ratio values achieved for PP3 are examined in detail below.
The Interpretation of the Strain Dependent Poisson's Ratio in Auxetic PP by Use of a Simple Nodule-Fibril Geometric Model
One successful technique for the interpretation of the strain dependent Poisson's ratio data in auxetic polymers has been to employ simple geometric models based on the nodule-fibril microstructure, which is known to produce the auxetic behavior (5, 11, 12, 20). This has allowed behavior in tension and compression for auxetic forms of PTFE and UHMWPE to be predicted and the mechanisms involved to be more clearly understood. This approach has now been applied to the strain dependent Poisson's ratio data (shown in [ILLUSTRATION FOR FIGURE 7 OMITTED]) obtained from mechanical testing of PP3 and, for completeness, the geometric model used is summarized below. Details are available elsewhere (12).
Summary of the Geometric Nodule-Fibril Model for Auxetic Polymers
The simple geometric model employed is shown schematically in Fig. 8a. It requires knowledge (usually obtained by SEM examination of the microstructure of the auxetic polymer under consideration) of the nodule dimensions, characterized by the ratio a/b where a and b are the major and minor lengths of the nodules, fibril length, and interconnectivity, characterized by the angle [[Alpha].sub.0], which is the angle between the fibril and the r axis. The model utilizes a space-filling unit cell that, from Fig. 8a, may be defined as having sides of:
r = 2a + 2[l.sub.1] cos [[Alpha].sub.0] (2)
z = 2b - 2[l.sub.1] sin [[Alpha].sub.0] (3)
As a compressive strain is applied, the angle between the fibril and the r axis varies, the angular variation being characterized by [Alpha]. Using this notation, the compressive engineering strain applied in the radial direction, [[Epsilon].sub.r], is given by:
[[Epsilon].sub.r] = r([Alpha]) - r([[Alpha].sub.0])/r([[Alpha].sub.0]) (4)
= [l.sub.1] (cos [Alpha] - cos [[Alpha].sub.0])/a + [l.sub.1] cos [[Alpha].sub.0] (5)
Now, from eq 1,
[v.sub.rz] = - [[Epsilon].sub.z]/[[Epsilon].sub.r] = ([Delta]z/[z.sub.0])/([Delta]r/[r.sub.0]) (6)
= - (sin [[Alpha].sub.0] - sin [Alpha])(a + [l.sub.1] cos [[Alpha].sub.0])/(cos [Alpha] - cos [[Alpha].sub.0])(b - [l.sub.1] sin [[Alpha].sub.0]) (7)
where [Delta]z = z - [z.sub.0] is a small, but not infinitesimal, change, defined as in the experimental method used; similarly for [Delta]r. As has already been stated, this is a very simple geometric model and so can only represent a first approximation to actual experimental behavior. However, despite this note of caution, good agreement between experimental data and theoretical prediction of strain dependent Poisson's ratio for both PTFE and UHMWPE has been obtained (5, 11).
Application of the Nodule-Fibril Geometric Model to Interpret the Data Obtained from PP3
Superimposed on the data for Poisson's ratio in Fig. 7 are theoretical curves calculated using Eqs 5 and 7. The geometric parameters a/b = 1/4 and [l.sub.1] = 0.1 a were obtained by a combination of SEM observation and modeling. The initial value of [[Alpha].sub.0] was assumed to be the most likely parameter to vary from sample to sample and curves for [[Alpha].sub.0] = 40 [degrees] to [[Alpha].sub.0] = 80 [degrees] and [[Alpha].sub.0] = - 60 [degrees] to [[Alpha].sub.0] = -80 [degrees] are shown in Fig. 7. All the data follow the trends indicated by the model, that of a decreasing magnitude for [v.sub.rz] as [[Epsilon].sub.r] increases. Both samples with a negative and positive [v.sub.rz] can be interpreted with the model.
In terms of extrusion, the positive [v.sub.rz] samples have resulted from overexpansion of the material after extrusion, leading to a negative [[Alpha].sub.0] value [ILLUSTRATION FOR FIGURE 8B OMITTED]. The highly sensitive variation of [v.sub.rz] to processing conditions is illustrated in Fig. 7. In a structure where a [less than] b and [l.sub.1] [much less than] b then quite small variations in the final volume of the extrudate will lead to very large variations in [[Alpha].sub.0] and hence in the Poisson's ratio.
The values of Poisson's ratio obtained for this microporous auxetic form of PP are small in comparison with those obtained for the equivalent (i.e. structural) form of auxetic UHMWPE (i.e. values of up to [v.sub.rz] = -0.22 at a strain of 0.016 for PP as opposed to [v.sub.rz] = -1.50 at the same strain for UHMWPE). This can be explained when the microstructures of the two auxetic polymers are observed. For UHMWPE, nodule dimensions are characterized by a/b = 1 i.e. the nodules are circular; for PP, the nodules are elliptical and these are modeled as being rectangular with a/b = 1/4. This process of reducing a/b causes a reduction in Poisson's ratio. So, if the Poisson's ratio of auxetic PP is to be increased to, say, approach [v.sub.rz] = 1, one method of achieving such an increase would be to start with more spherical nodules. Another approach would be to increase the expansion of PP in the radial direction, with the aim being to reduce [[Alpha].sub.0] to as close to [[Alpha].sub.0] = 0 as possible, representing full radial expansion. This latter is partially achieved for auxetic UHMWPE, with 0 [degrees] [less than] [[Alpha].sub.0] [less than] 40 [degrees] being the initial angles (12), whereas for PP, [[Alpha].sub.0] [greater than] 40 [degrees] , thus reducing the values of Poisson's ratio obtained.
Finally, SEM examination of the microstructure of auxetic PP has revealed that this polymer, when compared with the auxetic forms of both PTFE and UHMWPE, has a lower fibril density. The effect of this on the range of Poisson's ratio values attainable is not expected to be significant, though it may affect the values of modulus and strength (20).
A novel microporous form of PP has been fabricated via a thermal processing route, which displays auxetic behavior (i.e., has a negative Poisson's ratio). The starting point for the processing route, in common with that for auxetic UHMWPE, is a finely divided powder, the necessary characteristics of which have now been defined as an average particle size of less than 300 [[micro]meter] in conjunction with a rough surface. The exact degree of surface roughness has not yet been defined but a surface depth variation of at least 1 [[micro]meter] is observed. Processing via the thermal route employed here of PP powder with either a larger particle size or a smooth particle surface did not result in auxetic end-products.
A simple geometric model based on material microstructure and previously applied successfully to auxetic PTFE and UHMWPE was employed to aid in the interpretation of the strain dependent Poisson's ratio data measured for auxetic PP, providing a mechanism for the compressive Poisson's ratio of PP to be identified. From this model it can be seen that particle shape also determines the ease with which a well defined negative Poisson's ratio can be obtained. a/b ratios close to one are preferable to low a/b ratios and over expansion of the microstructure during processing leads to positive Poisson's ratios.
The authors wish to acknowledge the financial support of ICI Chemicals and Polymers (APP) and the EPSRC through the provision of a research associate-ship (KLA). KEE wishes to acknowledge the award of an EPSRC Advanced Fellowship during this work. The authors also thank D. McKerron and C. J. Spain of ICI for the supply of materials.
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|Author:||Pickles, A.P.; Alderson, K.L.; Evans, K.E.|
|Publication:||Polymer Engineering and Science|
|Date:||Mar 15, 1996|
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