Silane Cross-linked Polyethylene Research Articles

A kinetic study on the supercritical decrosslinking reaction of silane-crosslinked polyethylene in a continuous process

For the first time, a kinetic study on the supercritical decrosslinking reaction of silane-crosslinked polyethylene (S-XLPE) in a continuous process was conducted. The decrosslinking reaction in a continuous process was corresponding to the first-order reaction model in which the reaction rate was linearly proportional to the gel content. The residual gel content exponentially decreased with reaction time. Reaction constant was observed exponentially related to the reaction temperature. The continuous decrosslinking reaction revealed kinetic constants of 1.355 ± 0.103, 2.715 ± 0.177, and 4.348 ± 0.218 min−1 at 360, 380 and 390 °C, respectively and activation energy of 132.36 kJ mol−1.

Modified Crystalline Structure of Silane-Crosslinked Polyethylene in the Proximity of Nanodiamonds

The introduction of nanodiamond particles (NDs) in silane-crosslinked polyethylene is found to lead to a notable and systematic deformation of the polymer unit cell. X-ray diffraction evidence of the existence of a modified crystalline structure in the bulk of the polymer due to the presence of NDs is reported here for the first time. The covalent bonding between NDs and the surrounding macromolecular chains may support that the excessive local stress field ultimately distorts the polymer conformation, yielding a new distorted but still crystalline interface. Supporting data from solid-state NMR experiments confirm the existence of a modified crystalline interface of about 1–2 nm in all the nanocomposite materials.

Analysis of relaxation processes by mathematical modeling

The mechanisms of charge accumulation and relaxation in electrically insulating films of cross-linked polyethylene prior to and after thermal aging are studied using the method of thermostimulated depolarization currents. It is demonstrated that the stability of the electretic state in samples of silane cross-linked polyethylene is higher than that in samples of peroxide cross-linked polyethylene. The spectra of currents are analyzed by mathematical modeling. The mechanisms of charge relaxation in films of cross-linked polyeth-ylene are determined.

Single-sided Nuclear Magnetic Resonance for condition monitoring of cross-linked polyethylene exposed to aggressive media

The potential of single-sided Nuclear Magnetic Resonance (NMR) to monitor truly non-invasive changes in polymer materials during aging under aggressive media is for the first time evaluated. For this, the NMR method is used in combination with other condition monitoring methods including mechanical measurements, mass uptake, and differential scanning calorimetry. It is validated by studying for the first time the aging kinetics of silane cross-linked polyethylene (PEX) exposed to media used in oil and gas production and transportation, including aliphatic and aromatic hydrocarbons, sulphur solvents, and corrosion inhibitors in combination with CO2 and H2S. All investigated parameters changed, with the strongest effects detected for the NMR chain mobility and in the presence of hydrocarbons. Furthermore, a universal linear correlation curve could be established between the depression of the tensile strength and the chain mobility. This result represents a fundamental step towards establishing single-sided NMR as a new analytical tool for in situ condition monitoring of polyethylene working under sour conditions. The proposed approach can be easily extended to other polymer materials.

Silane Crosslinked Polyethylene from Different Commercial PE’s: Influence of Comonomer, Catalyst Type and Evaluation of HLPB as Crosslinking Coagent

In the present work, the synthetic route to obtain crosslinked polyethylene (PEX) was studied using vinyltrimethoxy silane (VTMS) as crosslink agent and commercial polyethylene (HDPE and LLDPE). Materials were processed with increasing amounts of VTMS (2.5, 5.0 and 7.5 wt.%). After insertion of silane onto polymer chain in solid state, samples were molded by compression. Gel content was used to measure the extension of crosslinking. Melting temperature and degree of crystallinity were evaluated using differential scanning calorimetry (DSC). The microstructure of the starting polyethylene was correlated with the effectiveness of crosslinking and the resultant thermal and mechanical properties of the PEX. Hydroxylated liquid polybutadiene (HLPB) was employed to improve crosslink efficiency, as well as mechanical properties of the material.

Structure and microbial diversity of biofilms on different pipe materials of a model drinking water distribution systems.

The experiment was conducted in three model drinking water distribution systems (DWDSs) made of unplasticized polyvinyl chloride (PVC), silane cross-linked polyethylene (PEX) and high density polyethylene (HDPE) pipes to which tap water was introduced. After 2 years of system operation, microbial communities in the DWDSs were characterized with scanning electron microscopy, heterotrophic plate count, and denaturing gradient gel electrophoresis. The most extensive biofilms were found in HDPE pipes where bacteria were either attached to mineral deposits or immersed in exopolymers. On PEX surfaces, bacteria did not form large aggregates; however, they were present in the highest number (1.24 × 107 cells cm−2). PVC biofilm did not contain mineral deposits but was made of single cells with a high abundance of Pseudomonas aeruginosa, which can be harmful to human health. The members of Proteobacteria and Bacteroidetes were found in all biofilms and the water phase. Sphingomonadales and Methylophilaceae bacteria were found only in PEX samples, whereas Geothrix fermentans, which can reduce Fe(III), were identified only in PEX biofilm. The DNA sequences closely related to the members of Alphaproteobacteria were the most characteristic and intense amplicons detected in the HDPE biofilm.

Reprocessable silane-crosslinked polyethylene: property and utilization as toughness enhancer for high-density polyethylene

Silane-crosslinked polyethylene (Si-XLPE) has been increasingly used as a replacement of polyvinyl chloride for use as residence water pipes and hydronic heating systems. Unfortunately, good management of Si-XLPE wastes originated in the manufacturing processes or from already used products is limited. This article presents a melt-recycling (remelting and remolding) of waste Si-XLPE and its use to produce the high-density polyethylene (HDPE) composition of improved thermal endurance, along with attractive balance of stiffness and toughness. Experiments were carried out with a series of Si-XLPE of different gel fraction and crosslink structure, either loose or tight, aiming at understanding the factors which dictate the reprocessability and properties of crosslinked waste. Tensile and impact properties, crystallization, and heat stability of the HDPE and waste blends were investigated. It is found that waste Si-XLPE materials of loosely network structure can be remelting and remolding as thermoplastic PE. Interestingly, the results showed that crosslink gel in the wastes could be as high as 70 %. Both tensile modulus and impact strength of the HDPE systematically increased with increasing loading and content of gel in the crosslinked materials. The addition of 50 % crosslinked waste led to a rise of heat distortion temperature of HDPE by 20 °C, without evitable change in the melting and crystallization temperatures.

Use of Single-Screw Extruder for Continuous Silane Cross-Linked Polyethylene Recycling Process Using Supercritical Alcohol

The development of a continuous process for supercritical fluids is desirable for high-pressure technology. In a previous study, it was revealed that a twin-screw extruder was applicable as the reactor and feeder for industrial supercritical fluid processes (Goto, T.; et al. Ind. Eng. Chem. Res. 2011, 50, 5661–5666). The advantages of twin-screw extruders include their conveying ability and flexibility of design, which make them ideal for research on the applications of extruder. However, twin-screw extruders are expensive because of their complicated gearbox, cylinder shape, screw, etc. Here we report that a single-screw extruder, which is less expensive and simpler than a twin-screw extruder, is also applicable as the reactor and feeder for commercial supercritical fluid processes. Moreover, the equipment for a single-screw extruder is more compact than that of a twin-screw extruder. The results of this study will promote the application of extruders in supercritical fluid engineering research and contribute to the industrialization of a wide range of supercritical fluid techniques.

Continuous Process for Recycling Silane Cross-Linked Polyethylene Using Supercritical Alcohol and Extruders

In this study, we developed a continuous process using extruders in a pilot plant for recycling silane cross-linked polyethylene (silane-XLPE) via chemical reaction in supercritical alcohol. As a first step, an autoclave was employed to reveal the effects of temperature and pressure on the reaction that selectively decomposes the siloxane cross-linking bonds. Selective decomposition of the siloxane bond was found to occur in the supercritical alcohol. In the next step, an extruder was used to continuously feed silane-XLPE into a tube reactor containing supercritical alcohol. Silane-XLPE was then extruded with an injection of supercritical alcohol. Alcohol remained in the supercritical state inside the cylinders of the extruder and the tube reactor. The recycled polyethylene (PE) was continuously extruded from the reactor at a rate of 14 kg/h. The product produced by this continuous process was the same as that from the autoclave. The mechanical properties of the recycled PE satisfied the requirements for ...

Integration of Pseudomonas aeruginosa and Legionella pneumophila in drinking water biofilms grown on domestic plumbing materials

Drinking water biofilms were grown on coupons of plumbing materials, including ethylene-propylene-diene-monomer (EPDM) rubber, silane cross-linked polyethylene (PE-X b), electron-ray cross-linked PE (PE-X c) and copper under constant flow-through of cold tap water. After 14 days, the biofilms were spiked with Pseudomonas aeruginosa, Legionella pneumophila and Enterobacter nimipressuralis (10(6) cells/mL each). The test bacteria were environmental isolates from contamination events in drinking water systems. After static incubation for 24 h, water flow was resumed and continued for 4 weeks. Total cell count and heterotrophic plate count (HPC) of biofilms were monitored, and P. aeruginosa, L. pneumophila and E. nimipressuralis were quantified, using standard culture-based methods or culture-independent fluorescence in situ hybridization (FISH). After 14 days total cell counts and HPC values were highest on EPDM followed by the plastic materials and copper. P. aeruginosa and L. pneumophila became incorporated into drinking water biofilms and were capable to persist in biofilms on EPDM and PE-X materials for several weeks, while copper biofilms were colonized only by L. pneumophila in low culturable numbers. E. nimipressuralis was not detected in any of the biofilms. Application of the FISH method often yielded orders of magnitude higher levels of P. aeruginosa and L. pneumophila than culture methods. These observations indicate that drinking water biofilms grown under cold water conditions on domestic plumbing materials, especially EPDM and PE-X in the present study, can be a reservoir for P. aeruginosa and L. pneumophila that persist in these habitats mostly in a viable but non-culturable state.

Decomposition of Silane-Crosslinked Polyethylene with Supercritical Alcohol

For the purpose of the development of a new recycling technique of thermosetting silane-crosslinked polyethylene (silane-XLPE) turned into thermoplastic polyethylene, the optimum conditions are determined to decompose crosslinking points consisting of siloxane bonds, while keeping the main chain consisting of carbon-carbon bonds in silane-XLPE using supercritical alcohols. As a result, more than 90% of the siloxane bonds are broken selectively and rapidly without the destruction of the main chain and silane-XLPE is converted to thermoplastic polyethylene using supercritical primary alcohols such as methanol at 320–360°C, 10 MPa in 20 min. On the other hand, in the case of supercritical secondary alcohol such as iso-propanol, around two thirds of the siloxane bonds did not decompose even at the high temperature region above 350°C. This is because it was difficult for bulky secondary alcohol to approach the crosslinking points inside the polymer. The reaction rates are measured for the decomposition of the crosslinking points with supercritical methanol and n-propanol. They are expressed by the first-order as well as the second-order reaction models well, because the alcohol concentration is much higher than that of the crosslinking points and can be considered to be constant during the reaction. Arrhenius plots of the second-order rate constants lay well on straight lines for both alcohols and the activation energies are 21.7 kJ/mol for supercritical methanol and 66.5 kJ/mol for supercritical n-propanol.

The influence of bovine serum lubricant on the wear of cross-linked polyethylene finger prostheses.

A two-piece finger prosthesis has been proposed, manufactured from silane cross-linked polyethylene. Using a finger wear simulator the prosthesis was tested in a dilute bovine serum lubricant. Five tests were undertaken, totalling over 45 million cycles of wear testing. In each test, a statically loaded control prosthesis was included. In all tests it was found that the lubricant uptake of the test components exceeded that of the control components, consequently no gravimetric wear was measured. To investigate this result further, six cross-linked polyethylene prostheses were taken and soaked in the serum lubricant at 37 degrees C. Three of the prostheses were statically loaded and the other three were unloaded. These tests lasted for over one hundred and sixty days. It was found that the lubricant uptake of the unloaded control components was greater than that of the loaded com-ponents. The test with statically loaded prostheses was repeated, firstly with distilled water and then in lubricant heated to 30 degrees C and 40 degrees C. No significant difference in weight increase due to lubricant uptake at these two temperatures was found. The weight increase due to soaking in dilute bovine serum was several times that due to soaking in distilled water. (Journal of Ap-plied Biomaterials & Biomechanics 2004; 2: 136-42).

Evaluation of Recycling Technology of Insulation of Cross-linked Polyethylene Insulated Cable using Supercritical Alcohol

The material recycling of the insulation of cross-linked polyethylene cable was studied. We successfully obtained thermoplastic recycled polyethylene from silane cross-linked polyethylene by using chemical reaction in supercritical alcohol.Here, the continuous process for the recycling of silane cross-linked PE using supercritical alcohol was constructed. The mechanical and electrical properties of recycled polyethylene satisfied the requirement of the cable insulation. These results indicate that the cable to cable and wire to wire recycling of silane cross-linked polyethylene will possibly be accomplished by supercritical technology using extruder.Moreover the environmental effect of this technology was evaluated by the amount of the carbon dioxide generated from the continuous process. These results indicate that recycling method using supercritical alcohol was useful for the reduction of the environmental pollution.

シラン架橋ポリエチレンの超臨界流体による連続的な熱可塑化プロセスの検討

電力ケーブルの絶縁材料などに使用されている．架橋ポリエチレン（架橋 PE）は分子間に架橋結合が導入されているので，廃棄物を再び溶融成形することが困難である．そのため現状ではマテリアルリサイクルが進んでいない．ここでは，超臨界アルコールで，シラン架橋ポリエチレン（シラン架橋 PE）を連続的に処理し，再生ポリエチレン（再生 PE）を得る方法を検討した．始めにバッチ式の装置でシラン架橋 PE の分解条件を検討した．この結果，アルコールを超臨界状態にした場合に架橋を形成するシロキサン結合を分解する反応が進み，架橋前の PE に近い構造の再生 PE を得られることがわかった．さらに，シラン架橋 PE を二軸押出機で押し出しながら押出機のシリンダーに超臨界アルコールを注入し，バッチ処理で得た最適条件と同様の条件を押出機の中で作ることを試みた．その結果，押出機中でアルコールの臨界点を超える 10 MPa 以上，320°Cの条件を達成し，連続的に再生 PE を得ることができた．押出機を用いて得られた再生 PE は，バッチ式で得た再生 PE と同等以上の特性を持つことがわかった．すなわち，超臨界流体中でのみ起こるシロキサン結合の分解反応が二軸押出機の中で再現できたと考えられる．これらの実験結果は，超臨界流体によって流動性の低いポリマーなどを化学的に処理するための連続式反応装置として，押出機が利用できる可能性を示している．

A comparison of the wear and physical properties of silane cross-linked polyethylene and ultra-high molecular weight polyethylene

Cross-linked polyethylenes are being introduced widely in acetabular cups in hip prostheses as a strategy to reduce the incidence of wear debris–induced osteolysis. It will be many years before substantial clinical data can be collected on the wear of these new materials. Silane cross-linked polyethylene (XLPE) was introduced into clinical practice in a limited series of acetabular cups in 1986 articulating against 22.225-mm alumina ceramic femoral heads and showed reduced wear rates compared with conventionally sterilized (gamma irradiation in air) ultra-high molecular weight polyethylene (UHMWPE). We compared the wear of XLPE manufactured in 1986 with the wear of UHMWPE manufactured in 1986 in nonirradiated and irradiated forms. In the nonirradiated forms, the wear of XLPE was 3 times less than UHWMPE when articulating against smooth counterfaces. The nonirradiated materials did not show signs of oxidation. In the irradiated forms, only UHMWPE showed high levels of oxidation, and this caused a substantial increase in wear. Antioxidants added to XLPE during processing gave resistance to oxidative degradation. When sliding against scratched counterfaces, the wear of UHMWPE increased by a factor of 2 to 3 times. Against the same scratched counterfaces, the wear of XLPE increased dramatically by 30 to 200 times. This difference may be attributed to the reduction in toughness of XLPE. Clinically, XLPE has been articulated against damage-resistant ceramic heads, and this probably has been an important factor in contributing to reduced wear. New cross-linked polyethylenes differ considerably from XLPE. This study indicates that it is prudent to examine the wear of new polyethylenes under a range of conditions that may occur in vivo.

A comparison of the wear of cross-linked polyethylene against itself under reciprocating and multi-directional motion with different lubricants

A two-piece, silane cross-linked polyethylene (XLPE) finger prosthesis has been developed. To further the knowledge of the wear of XLPE against itself, a number of ‘pin-on-plate’ wear tests were undertaken, under different conditions of lubrication. These were distilled water, bovine serum and dry conditions. A second group of tests were then carried out, in which multi-directional motion was applied to the test pins. All tests had XLPE pins loaded at 40N rubbing against XLPE plates. All the XLPE came from the same batch.In all tests, pin wear was much less than plate wear. Under reciprocation only, the least wear was found when bovine serum was the lubricant (k=0.6×10−6mm3/Nm) and the maximum wear was when distilled water was the lubricant (k=5.8×10−6mm3/Nm). When multi-directional motion was applied to the test pins, increased wear occurred under lubrication with bovine serum (k=3.4×10−6mm3/Nm). Surprisingly, wear decreased when distilled water was used (k=0.7×10−6mm3/Nm), yet wear factors remained similar in the ‘dry’ test (k=0.7×10−6mm3/Nm). The dry tests had remarkably low wear.

The wear of artificial finger joints using different lubricants in a new finger wear simulator

A new design of finger wear simulator has been manufactured. The simulator is a dual cycle machine, interspersing dynamic flexion–extension motion, where the loads were 10–15N, with periods of a static 100N ‘pinch’ load. Also, a two-piece finger prosthesis has been designed and manufactured from silane cross-linked polyethylene. Using the simulator, a comparison of the wear of the cross-linked polyethylene prosthesis was undertaken, with Ringer solution, distilled water and dilute bovine serum as the lubricants. Each test was run at 37°C and included a control prosthesis to account for lubricant uptake. All prostheses came from the same batch, having a gel content of 87%. In total, testing exceeded 27 million cycles. With bovine serum, the total wear factor for the prosthesis was 0.07×10−6mm3/Nm. With Ringer solution it was 0.98×10−6mm3/Nm and with distilled water the wear factor was 0.60×10−6mm3/Nm. This order of results matched that found with pin on plate wear tests using these same three lubricants. The lower wear found with bovine serum may have been due to the positive boundary lubricating effects of the proteins within the serum. Lubricant uptake was greater and more significant in the bovine serum test.

Silane cross-linked polyethylene for prosthetic applications Part I. Certain physical and mechanical properties related to the nature of the material

Certain physical and mechanical tests have been conducted and scanning electron microscopy carried out in order to characterize a high density polyethylene cross-linked using vinyl trimethoxysilane. It is suggested that this cross-linked polyethylene could be an alternative to high molecular weight polyethylene for prosthetic applications.