3 Pipe Forming Tools
3.4 Process-induced Microstructure and Properties
It is important to carefully characterise the microstructure in the pipes, because it is a key point for a correct understanding of their properties, as will be shown later.
The general concepts concerning polymer crystallisation and structure development in processing have been outlined in Section 2.4. The experimental results presented here concern the pipes introduced in Section 3.3.2. More detailed information can be found in [17-19] for PA12, and in [14] for PE.
3.4.1 Orientation
No semicrystalline morphology can be distinguished by optical microscopy in either the PA12 or the PE pipes. On the other hand, light microscopy can be used to characterise the global orientation by birefringence measurements. The identification of the extinction lines observed on polished sections of PA12 pipes (Figure 3.23a) reveals high birefringence values at the skin, decreasing rapidly from the external surface to the inside of the pipe. At the external surface, they are always greater than 0.018, while they are lower than 0.01 beyond the 100 microns below the external surface. Observation of thin microtomed sections confirms a considerable orientation of the external skin, which appears as a bright region in an overexposed picture
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(Figure 3.23b). The thickness of this skin varies from 11.6 µm to 23.3 µm and its birefringence is between 0.018 and 0.033. For given processing conditions, the level of birefringence depends on the polymer rheology, which is related to the molecular architecture. For instance, high birefringence values are observed in LLDPE (Figure 3.24), but not in LDPE and HDPE. Melting experiments have shown that birefringence is due to orientation and not to thermal residual stresses; the skin keeps its brightness up to melting. Thermal residual stresses would have relaxed at a lower temperature.
(a)
100 µm 20 µm
(b)
Figure 3.23 PA12 pipe (r, z) sections observed by transmission optical microscopy (r, radial direction and z, extrusion direction): (a) polished section (thickness about
200 mm) observed in monochromatic light (l = 546 nm); and (b) microtomed section observed in white light (thickness about 10 mm). Reproduced with permission from J.M. Haudin, A. Carin, O. Parant, A. Guyomard, M. Vincent, C.
Peiti and F. Montezin, International Polymer Processing, 2008, 23, 55. ©2008, Hanser [22]
The interpretation of birefringence in terms of orientation is confirmed by wide angle X-ray diffraction (see Section 2.4.3), which clearly shows crystalline orientation in the skin. In Debye-Scherrer patterns, the 001 ring of the PA12 structure presents intensity reinforcements perpendicular to the extrusion direction (Figure 3.25a). The level of orientation has been quantified by the 001 pole figure (Figure 3.25b). As in PA12 the chain axis is the b axis of the unit cell, these results demonstrate an orientation of the crystalline chains in the extrusion direction.
0
Figure 3.24 Birefringence profiles in the thickness of LLDPE pipes obtained for two extrusion line speeds: 8 m.min-1 and 20 m.min-1
Figure 3.25 Characterisation of the crystalline orientation in the external layer of a PA12 pipe: (a) Debye-Scherrer diagram; the extrusion direction is horizontal;
(b) 001 pole figure; ED - extrusion direction (z); TD - transverse direction (q); and ND - normal direction (r)
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The high level of orientation in the external surface layer, as well as its increase with extrusion line speed (Figure 3.24), can be attributed to the calibration step. The combination of quenching and mechanical drawing in the calibrator induces a plastic deformation of the outer layers. This statement is validated by the relation between birefringence of the pipe external skin and the parameter Drtank introduced in Section 3.3.3 (Figure 3.26). Logically, macromolecular orientation appears to be affected by the strength of the tube drawing through the calibrator: increasing Drtank leads to larger deformation at the pipe external surface, and to greater subsequent orientation.
4.0
3.5
3.0
2.5
2.0
1.5 1.0
1.25 1.75 2.25 2.75
Drtank
Skin birefringence (× 100)
3.25 3.5
Figure 3.26 Skin birefringence versus draw ratio in calibration tank
3.4.2 Crystallinity
Crystallinity in the pipe thickness can be measured in differential scanning calorimetry (DSC) experiments on thin slices cut out of the pipe at different depths. In PA12 pipes, three types of DSC traces are observed as a function of the location in the thickness (Figure 3.27). A single peak is observed in the skin region. For the intermediate layers (100, 200 and 300 µm from the outer surface), the main peak is preceded by
a shoulder, whose importance decreases with increasing depth. Finally, the internal region exhibits a specific behaviour: an exothermic peak is observed before the melting peak, which indicates that part of the polymer has recrystallised during heating in the calorimeter. This means that the crystallinity of these internal layers was initially lower (about 20 to 25% lower than in intermediate zones, whose crystallinity is in the 16-17% range).
Figure 3.27 Crystallinity analysis in the thickness of a PA12 pipe: (a) location and thickness of (q, z) cuts; (b) melting curves of the different cuts: (1) 0-25 µm;
(2) 100-125 µm; (3) 200-225 µm; (4) 300-325 µm; and (5) 500-1,000 µm.
Reproduced with permission from J.M. Haudin, A. Carin, M. Vincent, N. Amouroux, G. Bellet and F. Montezin, International Journal of Material
Forming, 2010, 3, 225. ©2010, Springer [19]
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These results can be understood by considering that crystallisation occurs under the combined effects of cooling and stress (drawing in the calibrator), according to the location in the pipe. In the skin zone, stress effects are very important, and lead to a significant orientation of the crystalline phase. In the intermediate region, orientation is weak, but stress effects seem to remain important enough to partly counterbalance the effects of cooling, which are weaker than in the skin zone. In the internal layers, there is no stress effect. Therefore, cooling predominates and quenching of the polymer leads to a lower crystallinity.
3.4.3 Surface State
In some processing conditions, surface analysis reveals defects on the pipes. These defects are aligned either parallel (z direction), or perpendicular (q direction) to the extrusion direction. During tensile testing, the defects along q grow (Figure 3.28) and generally lead to rupture.
Figure 3.28 Three-dimensional image of the surface defects along q after an elongation of 80% along z. Reproduced with permission from A. Carin, J.M.
Haudin, M. Vincent, B. Monasse, G. Bellet and N. Amouroux, International Polymer Processing, 2006, 21, 70. ©2006, Hanser [18]
The influence of the calibration parameters on the final surface state of the pipe is difficult to quantify. Indeed, the dimensions of the defects are the same for all the samples; no direct relation has been found between defect depth and calibration conditions. Only the number of defects varies from one condition to another. It seems to depend on the lubrication level in the calibrator and particularly on the lubricating water layer thickness.
3.4.4 Mechanical Properties
Figure 3.29 shows the relationship between elongation at break (eB) and birefringence of the pipe external surface (DnSKIN). Elongation at break significantly decreases as the skin orientation increases: eB rises from 120 to 222% when DnSKIN is reduced from 0.033 to 0.018. A significant initial orientation limits the ability of macromolecular chains to be stretched during tensile testing, and consequently reduces the elongation at break. Since the level of chain extension is maximum at the external surface after extrusion, these are the layers that limit the elongation at break.
100
1 1.5 2 2.5
Skin birefringence (× 100)
3 3.5 4
120 140 160 180 200 220 240
Elongation at break (%)
Figure 3.29 Elongation at break versus skin birefringence
The origin, number and arrangement of surface defects have also to be integrated into a general explanation of the origin of fracture. Performances of pipes are explained by the coupled influence of orientation and surface defects. Fracture is initiated by the surface defects, but a high level of orientation is necessary for the growth of initial defects. Thus, it is possible to optimise the elongation at break of the pipe by fitting the calibration parameters to reduce the calibrator draw ratio and increase the lubrication level, in order to limit the orientation and the number of defects.
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