Crystallization Of Ultrahigh Molecular Weight Polyethylene Research Articles

Non-isothermal crystallization kinetics of UHMWPE composites filled by oligomer-modified CaCO3

The processability of ultrahigh molecular weight polyethylene (UHMWPE) improved by oligomer-modified calcium carbonate (CaCO3) was observed in our previous work. In order to understand the effect of oligomer-modified CaCO3 on the crystallization of UHMWPE, the non-isothermal crystallization behavior and crystallization kinetics of UHMWPE composites filled by oligomer-modified CaCO3 was studied by differential scanning calorimetry in this work. Jeziorny and Mo methods were used to describe the non-isothermal crystallization kinetics of UHMWPE composites.The effect of modified filler content and cooling rate on the crystallization temperature and crystallization rate was discussed. The heterogeneous nucleation of modified CaCO3 slightly increases the crystallization temperature of UHMWPE. The crystallization enthalpy of UHMWPE composites is significantly higher than that of UHMWPE. The crystallization rate of UHMWPE composites depends on the filler contents and cooling rate.

Effects of cooling process on the solid–liquid phase separation process in ultra-high-molecular-weight polyethylene/liquid paraffin blends

Ultra-high-molecular-weight polyethylene (UHMWPE) microporous membrane is generally prepared by thermally induced phase separation process. The phase separation process is closely related to the cooling process in practical production. In this paper, the phase separation temperature of UHMWPE/liquid paraffin blends was explored by the hot-stage-optical device and differential scanning calorimeter (DSC), and the result showed that the temperature was about 105–125 °C. The effects of cooling condition and crystallization ability of UHMWPE on the separation process of the blends were also investigated by DSC. The results showed that the phase separation was affected by the cooling rates rather than the initial cooling temperature. And the crystallization of UHMWPE was mainly limited by the nucleating at a low cooling rate, which could form larger porous structure. In a word, it should be inspirational for the actual production of the UHMWPE microporous membrane. The solid–liquid phase separation will occur in the cooling process of UHMWPE/LP blends, which is driven by the external temperature difference. It is significant to understand the phase separation process. In this paper, the effects of the melting properties and cooling condition on phase separation process were investigated, the S–L phase separation range is at 105–125 °C, and the speed of phase separation is mainly related to the external temperature difference and the UHMWPE content. It could be inspirational for actual production of the UHMWPE membrane with more uniform pore structure.

In situ Polymerization of Nanocomposites by TpTiCl2(Et) System: UHMWPE Filled with Carbon Nanotubes

AbstractSummary: Nanocomposites of ultra‐high molecular weight polyethylene (UHMWPE) filled with multi‐walled carbon nanotubes (CNT) were obtained by in situ polymerization of ethylene by TpTiCl2Et. This novel catalytic complex activated with polymethylaluminoxane (P‐MAO) (Al:Ti = 200) allowed to incorporate the CNT at different compositions (0,1‐1 w/w %) into the UHMWPE matrix. The filler addition produced an important enhancement of the catalytic activity when it was compared to that of homogeneous ethylene polymerization carried out under the same experimental conditions (30 min; 1bar; 25 °C). This fact was attributed to in situ support of TpTiCl2Et onto the CNT surface, which not only could induce the stabilization of the catalytic system but also allowed the growing of polymeric chains around the CNT structures. The characterization of these nanocomposites was carried out by Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Thermal analysis showed that the incorporation of CNT (at the tested concentrations) did not produce changes in the polymer thermal stability, as revealed by the initial degradation temperature values. However, the CNT produced a nucleating effect in the crystallization of UHMWPE as observed by DSC, independently of the filler content. The crystallization temperature of the obtained nanocomposites increased and the crystallinity degree slightly increased as well.

Multi-scaled polymer-based composite materials synthesized by mechanical alloying

Multi-scaled composite materials are of great importance, because they exhibit higher mechanical properties than those attained using conventional fillers or polymer blends. In this work, multi-scaled composite materials based on ultra-high-molecular weight polyethylene (UHMWPE), quasicrystals, polyimide and bronze are investigated for use in the moving parts of machines, gears, bearings, and sliding elements. The main object is to investigate the process of fabricating such composite materials, and to check if these materials are reproducible and reliable to an industrial extent. The specimens were prepared using a high-energy planetary mill. When milled with bronze, the quasicrystalline phase was dissolved into an intermetallic solid solution; milling with polymers showed to conserve the quasicrystalline phase, whereas the crystallization of UHMWPE was achieved during the milling process. Tribological study of consolidated samples showed an increase in the wear resistance for the bronze-containing composite materials. In comparison with pure UHMWPE, the polyimide-based specimen exhibited higher strength and hardness. This work has demonstrated the possibility of producing composite materials with acceptable and reliable properties using the mechanical alloying technology.

Control of crystalline orientation to enhance the wear resistance of ultra-high molecular weight polyethylene crystallization cups for artificial joints

Ultra-high molecular weight polyethylene (UHMWPE) has been used as a bearing surface for artificial joints and its wear is one of the limiting factors for long term use of such prosthetic implants. Previous studies have revealed that the oriented and crystallized UHMWPE, prepared by plane compression, enhanced the resistance of wear. In this paper, we studied the influence of compression by half-spherical shape on the preferred orientation and wear. UHMWPE with slight cross-linking was compressed at molten state by using a sphere. A half-spherical cavity was molded, and the molded shape was similar to a cup. Wide angle X-ray diffraction (WAXD) tests revealed that the (1 1 0) and the (2 0 0) crystalline planes were orientated along the half-spherical cavity surface (3D orientation). The melting point, the heat of fusion, and the density near the curved surface showed relatively higher than a conventional UHMWPE. These structure and physical properties can fit to the results of plane orientation. Wear tests were performed using hip simulator, and the weight loss was compared to that of a conventional cup. In conclusion, the compression with the 3D orientation enhances wear resistance, similarly to the plane orientation results, and is useful for molding an improved cup.

Non-isothermal Crystallization of UHMWPE

Processing of Ultra High Molecular Weight Polyethylene (UHMWPE) parts involves non-isothermal cooling leading to crystallinity variations, which cause variations in the mechanical properties. Study of non-isothermal crystallization kinetics of UHMWPE forms the basis for process modelling. The crystallization of UHMWPE was studied at seven different cooling rates. The crystallization onset and peak temperatures were linearly related to the cooling rate. The crystallization of UHMWPE was concluded to be a nucleation dominated process with small contribution from growth of nuclei. Differences in ultimate crystallinity (≈11%) were produced due to different cooling rates. A significant portion of the change in ultimate crystallinity occurred at lower cooling rates (<6°C min−1). At higher cooling rates (6–22°C min−1) the change in ultimate crystallinity was insignificant.