Volume 42, Issue 4

Nowadays, numerous techniques are used to quantify the resistance of cellular polymers against a thermal load. These techniques differ in significance and reproducibility and are all dependent on foam density, structure (i.e., cell size and -distribution) and sample geometry. Very different behaviors are expected for extrusion- and bead foams, as well as for amorphous and semi-crystalline polymers. Moreover, established tests use temperature ramps which would lead to temperature gradients within the sample and thus to faulty results. In this study, we developed a new approach from an engineering perspective to minimize these influences. In this approach, the resistance against the thermal load is derived from a steady creep test with defined temperature steps under a mechanical load, which is specifically set for each foam sample depending on its static compression behavior at room temperature. The two-stage test therefore combines (i) a standard quasi-static compression test at room temperature and (ii) a creep test with stepwise increased thermal loading. For each foam type, a rather low mechanical load (stress) is determined from the quasi-static compression test at room temperature; low enough to remain below the collapse strength and avoid irreversible deformation (i.e., buckling and/or breaking of the cell walls). This load is then applied in a creep test where the temperature is increased in defined steps from room temperature to a temperature close to T g or T m . The stepwise increase and holding of the temperature for a defined time enables a homogeneous temperature in the test specimen. The approach was applied to (i) polystyrene extrusion and bead foams (i.e., XPS and EPS), which have different foam structure, (ii) amorphous and semi-crystalline bead foams of polystyrene (EPS) and polypropylene (EPP), (iii) bead foams with different densities (30, 60, 120, and 210 kg/m 3 ) and (iv) to a new type of bead foam made of the engineering polymer polybutylene terephthalate (E-PBT). The termination criterion for the test is defined as the temperature at which a relative compression of 10% is reached in the creep test with temperature steps. We suggest calling it the heat stability temperature T HS . For the studied foams, the procedure delivers characteristic T HS values that allow a good comparison between different polymer matrices and densities. The heat stability temperature T HS of amorphous PS foams (i.e., XPS and EPS) was determined to be 98 °C, which is close to the glass transition temperature T g . Using the same approach, values of 99–107 °C were determined for EPP and 186 °C for the semi-crystalline bead foam E-PBT.

The aim of the research was to identify the influence of different microencapsulated reaction time on the morphology, size, infrared spectral, thermal and micromechanical properties of melamine formaldehyde microspheres, synthesised with modified in situ polymerisation. Microspheres are microencapsulated particles with a blurred boundary of the core and shell due to their same composition. The synthesis of microspheres was paused after 1, 3, 9 and 15 h, and stopped after 23 h. The scanning electron microscopy and granulometric analysis were used to study the morphology and size of microspheres. Regardless of the reaction time, the produced microspheres were spherical in shape and with a rough surface. The average size of microspheres was almost identical (0.709–0.790 µm), while the volume size distribution curve of the particles became narrower with prolonged reaction time. The curing mechanism of melamine formaldehyde resin was studied using the Fourier-transform infrared spectroscopy and thermal analysis, and nano-indentation identification. The results revealed a slightly more crosslinked structure: with minimal (neglected) increased thermal weight loss (only up to 0.5%) and minor increased Young’s modulus (up to 2.3%). Using a nano-indenter, the hardness of synthesised particles improved by up to 14.8% after 23 h reaction time.

Hydroxyapatite (HAp) is a well-known bioceramic known for its high biocompatibility and good bioactivity. The structure of HAp mimics the natural bone structure and thus, it is widely used as implants for hard tissues. Despite possessing the above properties, it lacks mechanical strength, is susceptible to the growth of microbes over time and has low degradability. Polymers can be synthetic or natural. They can be a better choice to be used as additives to improve the properties of HAp due to its better mechanical strength and high biodegradability. A combination of metals and polymers together can overcome the drawbacks of HAp to a greater extent. This review article deals with different polymers and metal/polymer fabricated HAp to show the changes in the properties of HAp following the substitution. It also deals with how better they could be used as a hard tissue implant.

Trilayer polymer tubes were manufactured through an extrusion process using Pebax 6333 and high density polyethylene (HDPE) as outer and inner layer, respectively. A maleic grafted linear low-density polyethylene (LLDPE) was used as the bonding layer or tie-layer. Three types of multilayer tubes were produced: (1) outer layer (Pebax) at 70% of the total wall thickness (WT), (2) outer layer at 90% of WT, and (3) outer layer at 20% of WT. The analysis of mechanical properties showed that the inner layer contributes to strength and rigidity of the tube while the outer layer provides flexibility. Melt rheology behavior for HDPE and Pebax were studied, and HDPE showed a more pronounced shear thinning behavior compared to Pebax. Orientations of the tubes were assessed using Fourier transform infrared spectroscopy (FTIR) and it was found that HDPE layer is more sensitive to molecular orientation when extruded compared to Pebax material. Melting behavior for the tubes was investigated using dynamic scanning calorimetry (DSC). The tubes showed two melting temperatures: one associated with HDPE and one with Pebax. The HDPE peak showed a specific shift to a higher melting point for tubes as a result of molecular orientation during processing. Burst and compression testing were performed on the tubes and results demonstrated that the HDPE-rich extrusion showed the highest burst pressure and compression resistance.

In this article, isosorbide divalerate (SDV), an alternative renewable resource plasticizer for degradable poly(butylene succinate) (PBS) was successfully synthesized with isosorbide and valeric acid, and was characterized by Fourier transform infrared (FTIR). The mechanical properties, glass transition temperature ( T g ), crystallization properties, rheological behavior of PBS/SDV blends was studied in detail. The results showed that incorporation of SDV had successfully reduced T g of the PBS composites, particularly at 20 wt% SDV, where the value of T g exhibited a reduction of 12 °C or 39% compared to pure PBS, demonstrating SDV possessed plasticizing efficacy. The crystallinity of PBS was declined by presence of SDV in the blends, and the incorporation of 20 wt% SDV into PBS matrix promoted an impressive decrease of exceeding 22%. Significant enhancement of the toughness and flexibility of PBS was achieved by the addition of SDV. The rheological test revealed that the decrease of modulus and viscosity improved the processing properties of the materials, which broadened the PBS applications. Altogether the SEM showed the fracture surface of the composites undergoes a brittle-tough transition with increasing SDV content below 12% content, meanwhile, significant phase separation was observed in the composites with high content of SDV.

In this study, the application of methyl methacrylate (MMA) resin as the binder and standard sand as the aggregate has been employed to prepare the repair materials that can be cured in the sub-zero temperature environment. For this purpose, the redox initiation system of benzoyl peroxide (BPO) and N,N-dimethyl-p-toluidine (DMPT) has been used. Subsequently, the influence of initiator and accelerator content on the compressive strength, flexural strength, curing time and other properties of the materials has been revealed. At an ambient temperature of 0 °C, with BPO = 4.5% and DMPT = 3.5%, the developed repair materials can be cured within 31 min, and the 1 h compressive strength reaches 84.6 MPa. At an ambient temperature of −25 °C, with BPO = 4% and DMPT = 5%, the repair materials can be cured within 43 min, with the 1 h compressive strength reaching 53.4 MPa. The materials can be swiftly cured at low-temperature and exhibit excellent mechanical properties, thus, confirming their suitability for extreme environments. Fourier transform infrared spectrometry (FT-IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and other techniques have been employed to characterize the developed materials.

3D printed polylactic acid (PLA) composites reinforced with zinc oxide (ZnO) were successfully fabricated by attaching a ZnO dispenser beside a MendelMax RepRap Printer. Mechanical and electrical testings were performed to characterize the properties of PLA/ZnO and cPLA/ZnO composites. PLA polymer filament is a biodegradable material with the same strength and modulus as other plastic materials. This work emphasizes the effects of printing parameters (infill density and raster angle) on the mechanical properties and electrical conductivity of PLA/ZnO composites. The combination of 100% infill density and 0° raster angle resulted in a substantial increase in the tensile strength and Young’s modulus, but reduction in break elongation. As the infill density increased and shifted from 90° to 0° raster angle, the storage modulus was enhanced but the damping factor declined. Scanning electron microscope images proved ZnO was dispersed in the PLA matrix. Optical measurements showed a large air gap across the 35% compared to the 100% infill density. Conductive PLA composites (cPLA/ZnO) enhanced electrical conductivity when increasing infill density compared to PLA composites. Therefore, optimizing printing parameters could help manufacturers provide superior quality structure and cPLA composites for electrical and electronic applications due to good electrical conductivity.

Water-assisted injection molding (WAIM) is a promising molding process developed based on conventional injection molding (CIM). It has been a research hotspot in recent years and is still receiving extensive attention from many scholars and industries because of its significant potential advantages in practical applications. However, compared with CIM, since the additional water-related parameters are involved, the process moldability of thermoplastics is significantly reduced, especially for fiber-reinforced thermoplastics, which stunts the development of WAIM process. In this work, short-shot WAIM with an overflow cavity (OSSWAIM) was developed to address the problems and broaden the application scope of WAIMs. The results showed that compared with overflow WAIM (OWAIM) and short-shot WAIM (SSWAIM), OSSWAIM could significantly improve the process moldability and part quality of fiber-reinforced thermoplastics, especially for thermoplastic composites with a high fiber weight fraction. Besides, it was also found that water penetration had a slight influence on the fiber orientation near the water inlet, but had a significant influence on the fiber orientation near the end of mold cavity. Finally, three processing parameters affecting the water penetration, i.e., water pressure, melt temperature, and water injection delay time were investigated in terms of their influences on the fiber orientation within OSSWAIM.

This paper presents work from an ongoing study into the use of post-consumer waste polymer in the rotational moulding process. Raw plastic recyclate, predominantly consisting of polyethylene, was processed into polymer powders containing an additive package suitable for rotational moulding, with and without the addition of a compatibiliser. Rheological studies on the materials showed very high viscosities at low shear rates in comparison with materials typically used in rotational moulding, which has significant implications for melt flow and bubble removal during the process. It was possible to mould the materials successfully, but poor surface finishes were achieved. Impact strength was drastically lower for recyclate mouldings compared with virgin material. Tensile strength of the recyclate mouldings was reduced compared to virgin polyethylene, but results indicated that optimising the processing conditions could lead to improvements. This work gives some baseline indicators to inform further planned work on optimising processing conditions and identifying viable material compositions.