PA12 MFCs

Considering the tensile properties in general, another question about the influence of the chemical and physical structures arise. Based on the structure-properties investigations of the two polyamides in an oriented state [73,78], it would be expected to have a better performance of all PAG-reinforced composites, which was not confirmed in the experiments. The possible explanation will be looked for in the next subsection. 

Interestingly, a better mechanical performance of the PA 12 reinforced composites was observed, which was quite unexpected keeping in mind that some of the mechanical projrer- 

The first extensive SEM investigation of PA6/PET-based MFCs and their precursors performed by Evstatiev et al. [82] undoubtedly showed the fibrillar structure of the PET reinforcements preserved after the PA6 matrix isotropization. Since then, electron microscopy has been used to visualize the orientation and morphology of the matrix and reinforcing components in almost every report on MFCs. It is worth noting some more recent studies on MFCs comprising LDPE and PET as matrix and reinforcement, respectively [30,31]. Several microscopic techniques were used, e.g., SEM, polarizing light microscopy (PLM) and TEM. Thus, by SEM it was demonstrated that the isotropic LDPE matrix embedded PET microfibrils with random orientation. Thin slices of PLM and TEM showed the orientation in the machine direction. The latter method also revealed the formation of transcrystalline layers of LDPE on the oriented PET microfibrils. 

In regards to the studies on transcrystalhnity in conventional fiber reinforced composites, their number is vast. A number of issues are related to the formation and growth of TCL [81] crystallinity of the matrix, mismatch of thermal coefficients of the fiber and the matrix, epitaxy between the fiber and the matrix, surface toughness, thermal conductivity, treatment of fiber, etc. Processing conditions such as cooling rate, temperature, and interfacial stress were also found to be important. There are indications that the TC phenomenon is probably too specific for each fiber/matrix system. Nevertheless, it has been recognized that the orientation distribution of the polymer chains in the TCL wUl determine the nature and extent of its effect on the mechanical properties of the composite material [84]. 

There exist a limited number of studies on the occurrence of transcrystaUinity in MFC. Li et al. [38,42,43] studied the crystal morphology of iPP/PET in situ MFC, prepared by a slit extrusion, hot stretching-quenching process, and foimd that transcrystallinity occurred aroimd the PET in situ microfibrils. The authors propose different nucleation mech- 

---

In this chapter, results from the tensile, flexural and impact tests on HDPE/PA6 and HDPE/PA12 MFC are presented, studying the effects of the compatibilizer, HDPE and PA concentration, as well as the form and arrangement of the reinforcing entities on the mechanical behavior. The UDP MFC laminae were used for tensile tests. Impact strength and three-point flexural tests were performed on the CPC laminates. MRB and NOM composites were analyzed with the three mechanical tests. The data were compared with those of the neat HDPE matrix material and/or the oriented polyamide component [69]. 

Notes PA=PA6 or PA12 YP HDPE-MAH copolymer commercially available from DSM as Yparex UDP MFC lamina obtained from continuous oriented cables arranged in the form of unidirectional ply CPC MFC laminate obtained from cross-ply arranged oriented cables MRB MFC obtained from middle-length randomly distributed bristles NOM composite obtained from non-oriented mixture... 

Figure 14.12. Comparative impact tests of HDPE/PA6/YP and HDPE/PA12/YP composites (a)(b) Peak and Total energies of CPC MECs (c)(d) Peak and Total energies of MRB MFCs and (e)(f ) Peak and Total energies of NOM composites...

Figure 14.16. SEM images of cryogenic fractured surfaces of various HDPE/PA12/YP materials (compositions given in wt%) during the stages of the MFC preparation non-oriented blend after the extruder die (la-6a) slightly oriented blends after the first haul-off unit (lb-6b) MFC UDP, fractured in the direction of the fiber (lc-6c) [64]. AU the images are taken at the same magnification...

Figure 14.17. SEM images of various HDPE/PA12/YP samples after selective extraction of the matrix (a)s - final MFCs, and (b)s - non-oriented blends after the die exit with the following compositions (wt%) (l)s - 80/20/0 (2)s - 70/20/10 (3)s - 65/30/5 [64]...

Similar SAXS measurements were performed with PA12-reinforced UDP MFCs. Figure 14.24 shows the SAXS patterns of three representative HDPE/PA12/YP UDP MFC compositions (after corrections for the empty chamber scattering and for the intensity of the primary beam) (1) 80/20/0 without compatibUizer (2) 70/20/10 with compatibilizer and (3) 75/20/5 at different temperatures. It can be seen that at 30 C the three MFCs contain both isotropic scatterers, producing the circular reflection with an estimated Lg... 

Figure 14.24. 2D SAXS images of different HDPE/PA12/YP UDP MFC column (a) as prepared, at 30°C column (b) at 160°C, in-beam heating column (c) at 30°C after heating to 160°C (recrystaUiza-tion) [64]...

Table 14.9. Bragg s long spacing values of HDPE/PA12/YP UDP MFC at 30°C and after matrix recrys-taJlization (at 30 °C after melting at 160 °C) [64] ...

HDPE/PA12/YP UDP MFC composition (wt%) Isotropic scattering Oriented scattering Isotropic scattering Oriented scattering ... 

Both SEM and SAXS studies of UDP MFC materials reinforced by PA6 or PA12 fibrils gave evidence that the reinforcing fibrils most probably have a layered, coaxial structure a core of oriented PA and a shell of oriented, transcrystalline HDPE. The WAXS measurements supported and allowed a further development of this hypothesis. 

The same WAXS measurements were performed with the HDPE/PA12/YP unidirectional MFCs just like with the PAG-reinforced systems. Figure 14.30 displays 2D WAXS... 

Figure 14.30. 2D WAXS patterns of HDPE/PA12/YP UDP MFC taken at various temperatures. Fiber...

Figure 14.31. Peak-fitting of ID WAXS curve of the 70/20/10 UDP MFC at 160 C. The shaded peaks belong to a-PA12. The broad peak centered at 29 = 18° originates from the diffuse scattering of the amorphous material [64]...

Table 14.11. PA12 polymorph content in HDPE/PA12/YP MFC at 160 °C [64]...

To study the WAXS patterns of the as-prepared PA12-reinforced MFCs at 30 °C, trying to evaluate the TCL, the total scattered intensity was separated into two contributions using... 

Figure 14.32. Fraser corrected 3D WAXS patterns at 30 C after subtraction of the isotropic component of (a) 80/20/0 MFC UDP, and (b) 70/20/10 MFC UDP. The numbers indicate the following PA12 crystalline planes 1 - a(lOO) 2 - 7(020) 3 - 7(040) 4 - a(200). The fiber axis is vertical [64]...

Figure 14.33. ID WAXS profiles of the 70/20/10 HDPE/PA12/YP UDP MFC depicting the peak-fitting of the (a) oriented WAXS scattering and (b) of the isotropic WAXS scattering [64]...

Figure 14.34. Idealized model of the PA reinforcing fibril (a) representation of core, shell and TCL thicknesses, (b) and (c) to-scale representations of the cross-sections of the PA12 and PA6 reinforcing fibril in two HDPE/PA/YP MFCs 80/20/0 and 70/20/10. The solid circles represent the polyamide fibrils, and the dashed circles represent the transcrystaUine HDPE layer. For more details see 64 ...

Figure 16.13. MFC precursor blends from HDPE, two different polyamides (PA12, PA6) and a compatibi-lizer (YP) in tensile tests. Evolution of macroscopic stress and strain (cr,s-) as well as of nanostructure parameters. is the nanoscopic elongation computed from the HDPE long period. Di is the relative change of the width of the long period distribution. is the relative change of the extension of the microfibrils in transverse direction...

---