Volume 34, Issue 4

There is a growing literature investigating the effects of thermal annealing and green solvent exposure on the microstructural changes, in general, and transient structures in polyester fibers, in particular, using such techniques as birefringence, differential scanning calorimetry, X-ray diffraction, and dynamic mechanical analysis. The results obtained from these studies consistently indicate that transient structures in polyester fibers strongly depend on the medium used so that treatment in different media produces important changes such as the transformation of the intermediate domain into an amorphous or crystalline region. Compared to polyester samples annealed at high temperatures over long treatment times, those exposed to green solvents yield higher values of crystallinity, orientation, density, transient structure, crystallite size in lateral directions, and elastic modulus but lower values of fractional free volume. Being environmentally friendly, green solvents may, thus, be considered as superior replacements for the conventional materials used in thermal annealing, especially because they change the fine structure of polyester fibers over shorter treatment times and at lower temperatures.

Due to higher mechanical demands on technical parts, the application of short fiber reinforced thermoplastics for injection molding is strongly increasing. Therefore, more attention needs to be paid to the optimization of their recycling processes. Mechanical shredding of thermoplastics into granules is a common recycling method within polymer industries. The breaking of polymer chains and reinforcing fibers during this process may affect the material properties. This study presents the effect of ten recycling sequences on four different materials: polypropylene, glass fiber filled polypropylene, carbon fiber filled polypropylene and flax fiber filled polypropylene. Tests indicate that recycling has a negative influence on most of the mechanical properties. Polypropylene without fibers forms an exception as it does not exhibit any significant change in material properties. Glass fiber and carbon fiber reinforced polypropylene show a decrease in stiffness and tensile strength during the recycling steps. The impact strength of carbon and flax fiber reinforced polypropylene increases whereas that of glass fiber reinforced polypropylene decreases.

Self-reinforced polymer composites (SRCs), which are fabricated by combining the same type of polymer with different properties into one body, have high specific strength, no interfacial heterogeneity, and ease of recycling. To better understand the relationship between the molding process and mechanical properties of SRCs, the co-injection molding process was used in this study to process SRCs samples. Further, a self-developed visualized experimental device was used to observe the flow of the polymer melt directly during co-injection molding. From the visualization results, it was found that the tensile properties of SRCs are positively correlated with the absolute value of the slopes of the velocity change at lower melt temperature. When the melt temperature increases to a certain degree, the tensile properties of SRCs are much lower than that of lower melt temperature. The shear rate inside the melt flow and the shear rate between the melt and the matrix wall play a major role in the self-reinforcing characteristics of the SRC.

This paper explores the influence of structural parameters of ultrasonic transducer on the frictional plasticizing heat generation of polymer particles. A three-dimensional model of the interfacial friction plasticizing heating was established, and a transducer with different structural parameters was designed and analyzed. The actual output longitudinal vibration excitation of the transducer obtained by simulation analysis was directly loaded into the heating model, which further investigated the effect of various structural parameters on the frictional plasticizing heating process and temperature distribution of polymer particles. The results demonstrated that the interfacial friction plasticizing heating temperature of polymer particles increases nonlinearly with the longitudinal vibration excitation time, and the heat generation is a transient process. The amplification ratio has the greatest influence on the interfacial friction plasticizing heating rates of polymer particles, followed by both front cover length and piezoelectric ceramic pieces thickness; the effects of length of rear cover, horn and ultrasonic tool head are the smallest. The present work provides an effective basis for further studying the ultrasonic plasticizing of polymer particles in ultrasonic micro-molding.

It is well known that the process of injection molding with dynamic mold temperature control leads to a good replication quality of high aspect ratio microstructures. However, the inhomogeneous pressure distribution during the holding pressure phase results in an anisotropy of the component properties, low dimensional accuracy and, especially with optical polymers, in undesired stress birefringence. The anisotropy is based on the orientation of the molecular chains in the flow direction, which can be reduced by an injection-compression molding (ICM) process. In order to use the synergy from both processes, an injection-compression molding process with dynamic mold temperature control can be utilized. Within the scope of this investigation, the new process was reproduced by an ICM process at elevated mold temperature (ICM_EMT) and compared with injection molding (IM) with regard to molding accuracy and optical properties in dependence of component thickness and mold temperature. In order to evaluate the molding accuracy, the roughness of a wire-eroded microstructure on the cavity surface was measured. To determine the degree of orientation, the optical properties considered were the transmission and the path difference. It was shown that the adapted ICM process was able to achieve a high degree of replication accuracy with a low degree of orientation, especially for thin-walled components. ICM at elevated mold temperature reduced the path difference in the components with the lowest wall thickness by a factor of two while at the same time optimizing the replication of the microstructure. This could also be confirmed by transmission measurements.

Fused filament fabrication (FFF) is an additive manufacturing (AM) process that provides physical objects commonly used for modeling, prototyping and production applications. The major drawback of this process is poor mechanical property due to the porous structure of final parts. This process requires careful management of coalescence phenomenon. In this paper, the major influencing factors during the FFF processing of poly(lactic acid) (PLA) were investigated experimentally and with a numerical model. It has been shown that the polymer temperature has a significant effect on the rheological behavior of PLA, especially on the adhesion of the filaments. An experimental set-up has been placed in the machine to have the cyclic temperature of the filament. A variation of the polymer temperature influences process parameters such as feed rate, temperature of the nozzle and temperature of the platform. The results showed that the amount of polymeric coalescence (neck growth) rises when increasing the feed rate, the nozzle temperature, and the platform temperature. A model to predict the neck growth is proposed. It predicts a lower amount of neck growth value than obtained experimentally. This difference has been explained as the effect of other phenomena, such as polymer relaxation time, pressure of the nozzle and especially cyclic temperature which is not taken into account in the model.

This work focuses on the foaming behavior of thermoplastic vulcanized silicones (TPVs) in which partially crosslinked silicone nodules are dispersed. In these TPVs, silicone nodules dispersed in a low density polyethylene (LDPE) phase have an average size of about 1 μm. The crosslinking densities of the elastomer phase were selected according to their viscoelastic behavior. Surprisingly, linear and non-linear shear rheology appeared more sensitive to formulations than extensional rheology. Indeed, each formulation has an extensional rheological behavior similar to that of pure LDPE and meets the requirements for foaming applications in terms of elongation at break and melt strength. In accordance with non-linear shear rheology, the foaming behavior of these formulations has been correlated to extrusion foaming parameters that are known to control nucleation, i. e. pre-die pressure and die exit depressurization rate. With an appropriate crosslinking density of silicone nodules, the TPV foamability tends to the foamability of pure LDPE to reach a foam density of 0.54 g/cm 3 with an average cell size of 140 ± 50 μm and a cell density of 3 × 10 5 cells/cm 3 . Since partially crosslinked silicone nodules cannot foam, it is assumed that they improve nucleation while allowing sufficient expansion of the LDPE phase.

The ethylene-propylene-diene monomer/polypropylene (EPDM/PP) thermoplastic elastomer is prepared by dynamic vulcanization. The effects of different rubber ratios and crosslinking agent contents on the static and dynamic mechanical properties, crystallization and rheological behaviors are investigated and discussed. EPDM/PP with the rubber ratio from 40 to 50 % and crosslinking agent content of 1.2 wt% has the highest tensile strength and elongation at break. The fracture morphologies indicate that EPDM/PP with high rubber ratios and crosslinker contents is over vulcanized to harden, which results in that some large particles cannot be fragmented by shear force. The crystallinity obtained from differential scanning calorimetry (DSC) shows a decrease with increasing rubber ratio and a very slight variation with increasing the crosslinking content. The dynamic mechanical analysis (DMA) results display a decrease trend in storage modulus and tan δ with increasing crosslinking density. The rheological behaviors show that the complex viscosity also increases with increasing crosslinking density, indicating that the elastic deformation becomes superior to viscous flow in EPDM/PP.

Isotactic polypropylene (iPP) precursor film with highly oriented lamellar structure was produced through a melt extrusion-annealing-uniaxial stretching (MAUS) process. The precursor films were annealed at various temperatures and the structure evolution of precursor films during various annealing temperatures and the pore formation of the annealed film during stretching were investigated. During annealing, recrystallized lamellar structure was formed to obtain “core-shell sandwich” structure. The thickness of newly formed lamellae increased with annealing temperature, which improved the ability of slipping resistance of the original lamellar structure during stretching. As the annealing temperature of precursor film increased, the micropore shape and diameter of the final membranes became more uniform. The porosity of the microporous membranes increased with increasing annealing temperature.