Injection molded samples, morphology

The SEM micrographs in Fig. 19.7 show the morphology of longitudinal sections of the blends that are subsequently etched with formic acid. Since the exposed surface is parallel to the flow direction of the injection-molded samples, the micrographs are likely to reveal some process-induced orientation in the morphology. 

Skin/core morphologies are common in blends of LCP s and thermoplastic polymers and they play a significant role in defining the properties of both extruded and injection molded samples. Usually, LCP s in the skin have a higher degree of orientation than in the core when the blends are extruded or injection molded (Husman et al. 1980 Hedmark et al. 1989 Lee 1988). Baird et al. (Baird and Mehta 1989 Baird and Sukhadia 1993) observed a skin/core morphology in blends of PA 66 with HBA/HNA and 40 PET/60 PHB and 20 PET/80 HBA copolyesters. More LCP fibers were present in the skin than in the core for both systems. Isayev and Swaninathan (1994) also reported shell-core structure in the fracture surfaces of injection molded blends of HNA/HBA liquid crystalline copolyesters and poly (etherimide). 

Fig. 17.11 AFM images of PP/PP-g-MA/MMT/EOR nanocomposites showing elastomer particle morphology as a function of MMT content and PP molecular weight grade. The PP-g-MA/ organoclay ratio is 1.0 and elastomer content is 30 wt%. All images were taken from the core of the injection-molded sample and viewed parallel to TD elastomer particles are oriented parallel to FD (Tiwari and Paul 2011b)...

Thus, also in this case an increase in the quantity and quaUty of the available experimental data can help to remove ambiguity and aid to understand the polymer solidification in more detail. Interesting examples of the complex morphology that can be achieved in a transformation process come from the structural analysis of injection molded samples in Syndiotactic Polystyrene [22-25]. 

The morphologies of the injection-molded samples are consistent with the capillary extrusion results reported in Part I. 

Several mechanical tests were conducted on specimens with morphologies similar to that in Fig. 10.5, for several processing conditions. From the results of such tests, it has now been established that improvement in mechanical properties of TP/LCP blends can be correlated with the processing parameters. In particular, these studies have established that the relative thickness of the skin/core structure in the cross section of injection molded samples correlates with the magnitude of the mechanical properties. The latter, in turn correlates with the magnitude of three of the most critical process parameters in injection molding process, namely, injection speed, mold temperature and melt temperature. 

PP/EPDM Blends To improve impact properties of PP, the resin is usually modified by incorporation of an elastomer. Since the performance depends on morphology, radiation crosslinking was used in its stabilization [van Gisbergen et al., 1989a] (Table 11.9). The blends were made either in a two-roll miU, at 185°C, then compression molded into 1-mm-thick sheets, or in a co-rotating twin-screw extruder, then pelletized. The irradiations (100 kGy) were done using a 3-MeV electron accelerator. Irradiated and non-irradiated pellets were injection-molded. Eor the DSC measurements, the samples were melted at 200°C, quenched to 110°C, and then heated at a rate of 10°C/min. 

Flow field and cooling condition between injection molding and compression molding are essentially different. In particular, the flow-induced molecular orientation and/or the distorted shape of the dispersed phase have to be considered seriously in the injection molding. This part deals with the relation between morphology and mechanical properties in the injection-molded products for iPP and iPP/EHR blends (65). This is directly important for the industrial application. The characteristics of these samples are summarized in Table 9.5. 

Figure 4.10 Sample morphologies in injection moldings of Vectra under different injection speed. (Reprinted with permission from [14], copyright ( 1993 Butterworth-Heinemann Ltd.)...

The heterogeneous morphology and microphase separation process of block PLCs are closely related to the processing history and the molecular structures. For PET/PHB PLCs, PET-rich and PHB-rich phases are detected by SEM observation of etched samples [41,42]. When the PHB content is lower than 50 mol%, the PET-rich phase is continuous, and vice versa when the PHB content is higher than 60 mol%. The size of the PET-rich phase was found to be 10-20 pm with 40mol% PHB and 3-6 pm with 80 mol% PHB. In addition, the phase dimension will be influenced by the thermal and mechanical history. Joseph et al. have reported [41,42] that in an injection molded plaque of PET/PHB PLCs, PHB was richer in the skin while PET was richer in the core. However, there is controversy concerning this observation [3]. 

It is noted that the specific phase morphology of the injected molded tensile specimens used in the present study may differ from the compression molded film morphology. In the case of extruded samples of polymer blends displaying macrophase separation. Van Oene (25) indicates that the dispersed phase may appear as either ribbons (stratification) or droplets independent of shear strain rate but dependent of the post-extrusion thermal history. A study of the effect of morphology and phase inversion on the mechanical properties of the incompatible PPO blends is presently in progress. 

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