Microstructural origin of physical and mechanical properties of ultra high molecular weight polyethylene processed by high velocity compaction

Study into the Fire and Explosion Characteristics of Polymer Powders Used in Engineering Production Technologies

Polymers and their processing by engineering production technologies (injection, molding or additive manufacturing) are increasingly being used. Polymers used in engineering production technologies are constantly being developed and their properties are being improved. Granulometry, X-ray, FTIR and TGA were used to characterize polymer samples. Determination of the fire parameters of powder samples of polyamide (PA) 12, polypropylene, and ultra-high molecular weight (UHMW) polyethylene is the subject of the current article. An explosive atmosphere can be created by the powder form of these polymer materials, and introduction of preventive safeguards to ensure safety is required for their use. Although the fire parameters of these basic types of polymers are available in databases (e.g., GESTIS-DustEx), our results showed that one of the samples used (polypropylene) was not flammable and thus is safe for use in terms of explosiveness. Two samples were flammable and explosive. The lower explosive limit was 30 g·m-3 (PA12) and 60 g·m-3 (UHMW polyethylene). The maximum explosion pressure of the samples was 6.47 (UHMW polyethylene) and 6.76 bar (PA12). The explosion constant, Kst, of the samples was 116.6 bar·m·s-1 (PA12) and 97.1 bar·m·s-1 (UHMW polyethylene). Therefore, when using polymers in production technologies, it is necessary to know their fire parameters, and to design effective explosion prevention (e.g., ventilation, explosive-proof material, etc.) measures for flammable and explosive polymers.

Cyclic Impact Compaction of an Ultra High Molecular Weight Polyethylene (UHMWPE) Powder and Properties of the Compacts

Experiments on Cyclic Impact Compaction (CIC) of UHMWPE powder GUR 4120 were carried out on a laboratory hydro-pneumatic impact device. This device provides impact energies of up to 1 kJ with a frequency of impacts of 9 s^-1 and enables producing dense and robust compacts in the form of disks with a diameter of up to 60 mm and a height of up to 24 mm. The optimal parameters of the CIC were determined, which are the preheating temperature of the powder, the impact energy and the number of impacts. The strength, Brinell hardness and elongation of the resulting compacts with a diameter of 40 mm and a height of 15 mm were 37.5 MPa, 49.0 MPa and 470%, respectively. The possibility of activating UHMWPE powder by explosive loading was studied. It was found that the explosive pretreatment reduces the mechanical properties of the resultant compacts. The CIC method is suitable for the manufacture of UHMWPE-based composites with nano-additives, as evidenced by the successful production of compacts containing nanoscale detonation carbon as an additive. The results of the present study show that the CIC method is promising for the industrial production of small-sized UHMWPE parts.

Investigating the Feasibility of Processing Activated Carbon/UHMWPE Polymer Composite Using Laser Powder Bed Fusion

Activated Carbon (AC) is widely available at a relatively low cost, has a high porosity and is commonly used as a filter material for a range of applications. However, it is a brittle and friable material. Ultra-High Molecular Weight Polyethylene (UHMWPE) polymer is a tough engineering plastic that has been used as a binder. The traditional method used in manufacturing AC/UHMWPE filters involves compressing AC/UHMWPE composite powder during heating in a mould. This process compresses the particles together and the materials undergo sintering. This process results in a low pore interconnectivity, which has a considerable impact on the filter’s efficiency. Selective Laser Sintering is a laser powder bed fusion additive manufacturing technique for polymers. This has a number of advantages compared to the conventional technique and produces a porous structure with improved filtration efficiency. We propose that this is due to the greater pore interconnectivity. In this work, AC/UHMWPE powdered composites were prepared with different AC and UHMWPE ratios. The structure and properties of the AC/UHMWPE composite were investigated and characterised to assess their suitability for selective laser sintering. Particle size and morphology analysis were conducted, as well as density measurements, powder flow, thermal analysis, and crystallinity measurements. The results reveal that the addition of AC improves the UHMWPE flow. The thermal analysis results show that the intrinsic thermal properties of UHMWPE powder are not significantly affected by the introduction of activated carbon. However, thermal gravimetric analysis revealed that the onset of mass loss is considerably shifted (20 °C) to higher temperatures for the AC/UHMWPE composites, which is favourable for laser sintering. Additionally, the change in the composition ratio of untreated composite does not have a significant effect on the degree of crystallinity. Laser-sintered AC/UHMWPE parts were successfully manufactured using a commercial laser-sintering machine.

Enhancing Chain Mobility of Ultrahigh Molecular Weight Polyethylene by Regulating Residence Time under a Consecutive Elongational Flow for Improved Processability

Improving the processability of ultrahigh molecular weight polyethylene (UHMWPE) and understanding the effect of the polymeric chain mobility has long been a challenging task. Herein, we show that UHMWPE without any processing aids can be processed at a lower temperature of 180 °C compared to conventional processing temperatures (~250 °C) under a continuous elongational flow (CEF) by using an eccentric rotor extruder (ERE). By probing the effect of the residence time of UHMWPE samples under a CEF on the morphology, rheological behavior and molecular orientation, we find that the long polymer chains of UHMWPE are apt to orientate under a consecutive volume elongational deformation, thereby leading to a higher residual stress for the extruded sample. Meanwhile, the residence time of samples can regulate the polymeric chain mobility, giving rise to the simultaneous decrease of the melting defects and residual stress as well as Hermans orientation function with increasing residence time from 0 to 60 s. This also engenders the enhanced diffusion of UHMWPE segments, resulting in a defect-free morphology and higher entanglement with lower crystallinity but without causing obvious thermal oxidative degradation of UHMWPE. This interesting result could originate from the fast chain entanglement and particle welding enabled by a desirably short residence time, which could be explained by the empirical, entropy-driven melting explosion mechanism.

A review on high velocity compaction mechanism of powder metallurgy

High Velocity Compaction (HVC) is a key technology to realize the performance of powder metallurgy parts "three high and one low" (high density, high precision, high performance, and low cost). The main purpose of this paper is to summarize the research results at home and abroad on the forming mechanism, forming influencing factors and theoretical basis of high velocity compaction. This paper analyzes and summarizes the literature and monographs at home and abroad through literature research methods to obtain the research results of high velocity powder metallurgy compaction, so as to fully and correctly understand the research progress, development trends and application prospects of high velocity compaction. This paper expounds the technical characteristics, advantages and limitations of high velocity compaction, and analyzes and looks forward to its development trend, application prospect and expansion field, So as to provide reference and reference for the future research and application direction of high velocity compaction technology.

Extraordinary mechanical performance in disentangled UHMWPE films processed by compression molding

An approach to obtain disentangled ultra-high molecular weight polyethylene (UHMWPE) films is proposed using a common compression molding. For that, disentangled UHMWPE nascent powders from reactor are processed at temperatures lower than the main melting peak and at high pressure. Then, disentangled UHMWPE films obtained from homogeneous polymerization powders and from those that incorporate SBA-15 mesoporous silica can be easily achieved by this simple methodology. These disentangled UHMWPE based materials show very high crystallinity and, consequently, outstanding elastic modulus and hardness, both further increasing by presence of mesoporous SBA-15 in the hybrids.

Study on Permeability of Porous Ultra-High Molecular Weight Polyethylene (UHMWPE)

In recent years, porous materials are gaining in popularity for engineering applications, due to their special characteristics, such as low density, large specific surface area, and excellent permeability. In this study, powder processing technique was used to prepare ultra-high molecular weight polyethylene UHMWPE porous materials. Sintering temperature was obtained by combining differential scanning calorimetry (DSC) analysis and tensile tests. The surface morphology of sintering necks and tensile fracture were observed by scanning electron microscopy (SEM). Finally, single factor tests and orthogonal experiments were conducted to optimize three main processing parameters for a better permeability. It is found that the proper sintering temperature range would be from 143 to 153.1°C. According to the significance of influence, processing factors come in the sequence of the particle size, the compaction strength and the sintering temperature. Porous materials were successfully prepared, under the optimized parameters as the particle size >250 μm, the compaction strength of 2.5 MPa and the sintering temperature of 152°C.