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Ogila et al. – eXPRESS Polymer Letters Vol.11, No.10 (2017) 778–798 environmental friendliness and to some extent bio- degradability. They are, however, being held back by their high cost and brittleness. Also, due to their high zero shear viscosity RM of these polymers is very challenging. Greco and coworkers [78, 107] investigated the sin- terability of a PLLA and its suitability to RM. Ex- perimental results enabled the formulation of three dimensionless numbers G1, G2 and G3; which were representative of the onset of sintering, end of sin- tering and onset of degradation respectively. The evo- lution of these numbers generated Continuous Heat- ing Transformation (CHT) curves; which showed that a processing window wide enough for RM could be achieved only with the application of slower heat- ing rates. Further investigations into the effect of plasticizer showed a substantial reduction in the tem- perature of coalescence (Tcoalesce) due to reduced vis- cosity; this was, however, accompanied by a drastic lowering of the temperature at the onset of degrada- tion (Ton, deg) by almost 100 K. No apparent correla- tion could be found between the type of plasticizer and onset of degradation. 5.4. Liquid plastic systems The liquid plastics systems (LPSs) applied in RM fall into two distinct groups: reactive and non-reac- tive. Reactive LPSs undergo in-situ polymerization in RM and require catalysts and or activators to enable reasonable reaction times. These reactive processes, give rise to the term reactive RM (RRM). In RM re- active LPSs can further be classified, according to their polymerization mechanisms as: thermosetting (TS) systems, which include silicones, polyurethanes (PU) and unsaturated polyesters (UP); and thermo- plastic (TP) systems which are made up predomi- nantly of caprolactam (CL) [1, 108], and to a lesser degree laurolactam [109, 110]. The polymerization reaction for TS LPSs occurs under the action of a catalyst at either the functional end groups or by the opening up of double bonds along the polymer backbone [1]. The process results in the formation of intractable three dimensional crosslinked polymer networks. TS systems such as silicones and UP may require some heating; while highly reactive systems such as PU are usually re- acted in a cold mold. The in-situ RRM of lactams, which make up the re- active TP group of LPSs, involves the ring opening polymerization of a low viscosity precursor by the action of heat, chain initiators and catalysts. This process occurs in three distinct steps: (i) the formation of anions, (ii) the formation of a complex from the combina- tion of activator and catalyst and (iii) polymerization through the action of anions, with new anions formed during the addition of each consecutive monomer [111]. The non-reactive classification for RM LPSs is re- served for a single polymer system, PVC plastisols. PVC plastisols are manufactured by suspending 0.1– 0.2 micron sized particles in a plasticizer [1]. During processing via RM, the polymer system undergoes a number of changes with increasing mold tempera- ture. As the glass transition temperature of the poly- mer approaches, the PVC particles absorb the plas- ticizer and begin to swell. This continues until all the plasticizer has been absorbed. At this stage the poly- mer that is deposited on the mold wall lacks cohe- sion between its particles and is not structurally stable. Fusion begins when the PVC reaches temperatures of approximately 120 °C; and when temperatures in- creases further to 190 °C the PVC plastisol becomes fully densified [112]. The processing of liquid systems by RM is less te- dious than that of solid plastics. This is because, un- like the latter, liquid RM is characterized by; a short heating duration (approximately 15 minutes) with lit- tle cooling required, a low material initial viscosity, and a wider selection of applicable materials. LPSs have a long history of application in RM; beginning with PVC plastisols since the early 1940s, followed later by caprolactams and PU in the 1950s and 1970s respectively [113, 114]. Subsequent works investi- gated and attempted to resolve the problems inherent to these materials, such as uneven material distribu- tion and part porosity [108, 115–120]. Throne and Gianchandani [120], identified four flow regimes for a reactive polyester resin: cascading, rim- ming, stable hydrocyst, and solid body rotation (SBR) (Figure 7). During cascading flow a volume of poly- mer left the pool at the bottom of the mold and at a certain rotational angle flowed back down. With in- creasing time, the rise in viscosity due to polymer- ization caused the polymer volume that left the pool to stick to the mold wall, giving rise to the rimming flow. Increased viscosity gave rise to rings of mate- rial also known as hydrocysts; which, due to rising viscosity, disappeared after a short while leaving a solid body rotation (SBR). The authors determined 791PDF Image | Rotational molding: A review
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