Polymeric materials densification

One more important aspect is necessary to note. From the Eq. (14.3) it follows, that in any case 6F 6F. The opposite sigh of the inequality (v < v) means volume decreasing or polymeric material densification that is impossible [6]. 

All the physical ageing phenomena can be understood as the slow relaxation of the configurational entropy that is frozen into the glass on quenching below Tg. In the case of the simple system ort/zo-terphenyl, the ageing process goes to completion and crystals are formed. With most polymeric materials the development of order is limited and densification is the best description of the process. An extensive review of this topic is to be found in Struick s book. ... 

Hydrolysis and Polycondensation. As shown in Figure 1, at gel time (step C), events related to the growth of polymeric chains and interaction between coUoids slow down considerably and the stmcture of the material is frozen. Post-gelation treatments, ie, steps D—G (aging, drying, stabilization, and densification), alter the stmcture of the original gel but the resultant stmctures aU depend on the initial stmcture. Relative rates, of hydrolysis, (eq. 2), and condensation, (eq. 3), determine the stmcture of the gel. Many factors influence the kinetics of hydrolysis and... 

During compression of polymeric foams, three characteristic stages of deformation are commonly observed. At low deformations, the polymer foam is in the linear elastic response regime, i.e., the stress increases linearly with deformation and the strain is recoverable. The second phase is characterized by continued deformation at relatively constant stress, known as the stress collapse plateau. And the final phase of deformation is densification where the foam begins to respond as a compacted solid. At this point the cellular structure within the material is collapsed, and further deformation requires compression of the solid foam material (Ouellet et al. 2006). As mentioned above, a specific technique is required to obtain stress-strain curves of ferroelectrets in thickness direction because the thickness in ferroelectrets is normally very thin, corresponding to very small defiections. Dansachmiiller et al. developed an experimental technique that allows obtaining the stress-strain curves in ferroelectret films (Dansachmiiller et al. 2005). This method may also be used to obtain the stress-stain curve for a polymer foam film without oriented macro-dipoles. The schematic of the experimental setup is shown in Fig. 4. 

There are several designs for the feed mechanism. The simplest one is the use of a plunger which pushes the material into the mold. However this gives rise to densification of die material. In another design the mix is forced from a cylindrical tank down to the entrance of a screw injection cylinder. The screw conveys the material to the mold at the fixmt of the machine. Short screws are used for thermoset resins, and the compression ratio is below 1.5. Otherwise, heat may be produced and hardening may take place inside the machine. The barrel of the machine needs to be cooled to prevent premature polymerization. The typical temperatures at the rear and front of the cylinder are around 50 and 65"C respectively. 

Figure 16 Densification of materials in powder-based RP processes, (a) Loosely arranged powder particles prior to solidification (b) porous green part that can be postprecessed by (c) infiltration with polymeric resin (d) isostatic pressing or (e) sintering or pyrolysis followed by infiltration with low melting alloys.

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