Tensile drawing

3 Solid Phase Deformation Processes 3.1 Tensile Drawing 

Secondly, it was shown that polypropylene 57) and polyoxymethylene 58) could also be drawn to high draw ratios and high Young s moduli. Essential ingredients were 

The early studies of Capaccio and Ward 2,45,4 5), which led to the discovery of high modulus polyethylene, were based on drawing low molecular weight polyethylene homopolymer at comparatively low temperatures. Subsequent investigations were addressed at three major extensions of this work  

1) The drawing behaviour of LPE homopolymers with a very wide range of molecular weights. 

2) Examination of the effect of temperature on drawing behaviour and Young s modulus of drawn polymers. 

These preliminary results were confirmed and extended dramatically in subsequent investigations by Capaccio, Ward and co-workers A key step was the re- 

---

Elenga, R., Seguela, R. and Rietsch, F., Thermal and mechanical behaviour of crystalline polyethylene terephthalate) effects of high temperature annealing and tensile drawing, Polymer, 32, 11, 1975-1981 (1991). 

The same molecular mechanisms as in tensile drawing are observed, of course, in constant load experiments. Depending on the stress-time-temperature regime essentially four different failure modes are observed with thermoplastic materials ... 

The next major development takes up the idea of the gel layer, and combines this with the tensile drawing of fibres at high temperature, hopefully to give a preparation route which is more acceptable in terms of production rates. We therefore now have a two stage process, in which a fibre of suitable initial structure is first produced, followed by a hot stretching process, and historically two parallel accounts have been given more or less contemporaneously. Smith and Lemstra describe the... 

In a recent investigation the tensile drawing behaviour of a range of polyethylene copolymers has been studied. Full details of the copolymer composition, including the molecular weight characteristics are given in Table 2, which gives details for all polyethylene copolymers discussed in this paper. Dumbbell samples cut from quenched sheets were drawn at 75 or 115 °C at a standard crosshead speed. It was found that even at the very low concentration of one side branch per 1,000 carbon atoms there was a... 

The full potential of the hydrostatic extrusion technique became apparent in 1974, when the production of ultra high mudulus polyethy lenes with stiffnesses up to 60 GPa were reported The main process parameter in hydrostatic extrusion is the nominal extrusion ratio Rj, the ratio of the billet cross-sectional area to that of the die exit (assuming deformation occurs at constant volume, which is a very good approximation). Because polymers can exhibit die swell in extrusion, it is convenient also to define an actual extrusion ratio R, based on the ratio of the initial and final billet cross-sections. R is, of course, direcUy comparable to the draw ratio in tensile drawing (assuming plug-flow) and in practice R R for all but the lowest reduction ratios. 

It was considered initially that because there is no net tensile stress in a hydrostatic extrusion process it might be possible to impose very large plastic deformations without incurring fracture. It was indeed shown that Rj, of 30 can be imposed in polyethylene comparable to the draw ratios of 30 adiieved in a tensile drawing process, and that both processes are limited by the strain hardening behaviour of the material, which is determined solely by the total plastic strain imposed. This led to an important... 

The Young s moduli of the small-diameter extrudates were uniquely related to the extrusion ratio R to a very gocxl approximation. As shown in Fig. 19, this relationship does not depend on the molecular weight of the polymer, consistent with the second principle enunciated above. In fact, it appears from extensive studies of the structure and properties of oriented LPE, PP and POM that comparable materials are produced in large section by hydrostatic extrusion to those produced as fibres or tapes by tensile drawing. 

The polymer is not contact with the die wall, and draws as in free tensile drawing, with a neck. The polymer is contained within the heated die, but it is likely that the drawing process is partly controlled by adiabatic heating of the polymer in the necked region. 

It is only in the case of high modulus polyethylene produced by tensile drawing or hydrostatic extrusion that extensive structural studies have been undertaken. For these materials it was found in agreement with previous studies by Peterlin... 

The carrier generation mechanism in PPV has been addressed by studying the transient photoconductivity and the photoluminescence as a function of the external electric field, E, in samples oriented by tensile drawing [166]. The transient photocurrent is proportional to E at low fields, but increases nonlinearly for E> 10 V/cm. The field at which the photoconductivity becomes nonlinear (the onset field, EP ) depends on the degree of alignment the higher the draw... 

The most common type of uniaxial extension can take place during simple tensile drawing of cast polymeric sheets. In this case, the two ends are gripped and the material is pulled to extend, and then chain extension and crystallite rearrangement takes place. 

The effect of tensile draw on the "cluster" reflection is shown in Figure 12 and should be compared with Figure 7 for unhydrolyzed polymer. Under moderate strains, 1.75 times, it is seen that the "cluster" reflection is observed mostly in a direction normal to the strain. This is also true in the SAXS scan from fiber where the reflection is principally observed in the equatorial scan. This implies a periodicity which is normal to the fiber axis. Thus, the periodicity associated with this reflection tends to be orthogonal to the periodicity associated with the reflection observed in the meridional SAXS scan of unhydrolyzed fiber. Similar effects of sample orientation have recently been reported for ethylene-methacrylic acid ionomer (23). 

---