Is High-density Polyethylene (HDPE) a Good Choice For Potable Water
High Density Polyethylene
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A brief explanation of the Phillips process is provided in the following paragraph. The process shown above represents Phillips Petroleum Co. suspension ethylene polymerization in 1961. The polymer particles are suspended in an inert hydrocarbon. The melting point of high-density polyethylene is approximately 135 Celsius. Therefore, slurry polymerization takes place at a temperature below 135 Celsius; the polymer formed is in the solid state. If the polymerization were to take place at a temperature greater than its melting temperature then the polymer formed would be in the liquid phase. The Phillips process takes place at a temperature between 85-110 Celsius. A loop reactor is used in a liquid-phase process. The catalyst and the inert solvent are introduced into the loop reactor where ethylene and an -olefin are circulating. The inert solvent is used to dissipate heat as the reaction is highly exothermic. A cooling jacket is also used to dissipate heat. The active sites on the catalyst are equally accessible to the monomer throughout the particle. Therefore, the polymer chains grow not only outwards but also inwards, causing the granule to expand progressively. The reactor consists of a folded loop containing four long runs of pipe 1 m in diameter, connected by short horizontal lengths of 5m. The slurry of HDPE and catalyst particles circulates through the loop at a velocity between 5-12m/s. The reason for the high velocity is because at lower velocities the slurry will deposit on the walls of the reactor causing fouling. The concentration of polymer products in the slurry is 25% by weight. Ethylene, alpha olefin comonomer (if used), an inert solvent, and catalyst components are continuously charged into the reactor at a total pressure of 450 psig. The pressure is a lot higher than the pressure used to create high-density polyethylene by the Ziegler process. The high pressure creates HDPE polyethylene with fewer branches than the HDPE created by the Ziegler process. The HDPE created by the Phillips process typically has one ethyl branch per every 100 molecule chains while HDPE created by the Ziegler process has three ethyl branches per every 100 molecule chains. Because of this, the density of high-density polyethylene created by the Phillips process is higher. This has its advantages in processing. The HDPE created by the Phillips process is more crystalline and it is used to create more durable products. The polymer is concentrated in settling legs to about 60% by weight slurry and continuously removed. The solvent is recovered by hot flashing. The polymer is dried and pelletized. The conversion of ethylene to polyethylene is very high (95%-98%), eliminating ethylene recovery. The molecular weight of high-density polyethylene is again controlled by the temperature of catalyst preparation (too high of temperature increases spontaneous chain transfer, but increases the rate of reaction). The goal of the engineer is to find the temperature that optimizes the process. The molecular weight can be controlled by the addition of hydrogen into the reactor. Chain transfer will then occur. The following chain transfer reactions possibly could take place in the production of HDPE by the Phillips process:
The labelling with radiometals can be direct or chelator-mediated (tagged). The direct 68Ga-labelling of macromolecules is limited and applies to proteins (e.g. lactoferrin, transferrin, ferritin) designed by nature for iron binding thus utilizing chemistry similarity of Ga(III) and Fe(III) . The direct 68Ga-labelling and formation of low molecular weight complexes is commonly employed for the development of imaging agents for perfusion or for imaging of biological processes where the agent uptake is defined by its charge, lipophilicity, and size. Particulate agents can also be produced by the direct 68Ga-labelling either by co-precipitation (e.g. macroaggregated albumin) or by co-condensation (e.g. 68Ga-carbon nanoparticles) [-]. The chelator mediated 68Ga-labelling, requiring first synthesis of a bioconjugate comprising vector molecule and chelate moiety for the coordination of the radiometal ion, is the most common pathway of imaging agent design. The principle components of such agents are targeting vector, chelator, and radionuclide (Figure A). The modulation of pharmacokinetics, biodistribution, and stability can be achieved by the introduction of pharmacokinetic modifiers (PKM) such as hydrocarbon chain, polyethylene glycol (PEG), carbohydrate, and polypeptide chain. PKM may also serve as linker/spacer between the bulky chelate moiety and the active site of the vector molecule. Thermodynamic and kinetic stability, geometry and lipophilicity of a chelator-metal ion complex are important parameters in the development of radiometal based radiopharmaceuticals.
Ultra-high-molecular-weight polyethylene - Wikipedia
How did this polymerization occur? The question is still debated today. The mechanism of the reaction to create high-density polyethylene is still not fully understood today. The question that persists is where the active site of the polymerization takes place. Today, the theory put forth by Natta is most credibly believed. Natta worked alongside Ziegler in a laboratory in Germany. He created polypropylene by reacting an organometallic with a metal alkyl to create an active site for polymerization. After creating the catalyst, Natta introduced propylene to create polypropylene. The catalyst created is very similar to the catalyst Ziegler created to produce high-density polyethylene. Because of this, the catalyst’s used today to create polyethylene and other types of polymers are referred to as the Ziegler-Natta catalyst. Natta explained the mechanism by explaining that the reactive site of the catalyst is the Ti-C bond and not the Al-C bond formed during the initiation step. The following reactions show the initiation step that creates the active site for polymerization, and the propagation step for the production of polyethylene:
The idea of the mechanism involves catalyst formation and chain propagation in which the Chromium d-orbitals interact with the electron cloud of ethylene. The mechanism is classified as anionic polymerization or "living" polymerization. Living polymerization means the polymerization continues until the concentration of ethylene runs out. Because of this, the molecular weight of the polyethylene created can be extremely high. One way to control the molecular weight of high-density polyethylene created is through chain transfer reagents. Some typical chain transfer reactions are shown below:
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The results show the greatest yield is when the ratio is when the Al/Ti ratio is 0.9. The molecular weight continues to rise over an Al/Ti ratio of 2.0, however, the weight average remains constant. The Ziegler process produced a high-density polyethylene at pressures as low as atmospheric pressure. High-density polyethylene is a more durable polymer when compared to low-density polyethylene due to its lower degree of branching.
A brief description of the Ziegler process will be explained in the following paragraph. First, the organometallic compound (i.e. titanium tetrachloride) is reacted in a reaction vessel with a metal alkyl at a temperature between 100-130 degrees Celsius in the presence of a solvent. The pressure of the reaction vessel is between atmospheric and 20 atm. Ethylene is introduced into the reactor vessel in the gas phase. The boiling point of ethylene is approximately –100 degrees Celsius. The ethylene reacts with the active site of the catalyst to produce polyethylene. The solvent is used to dissipate heat. The solvent must not vaporize or react with any of the compounds in the reactor (inert solvent). The melting point of high-density polyethylene is approximately 130 degrees Celsius. Therefore, the polyethylene formed is in the solid phase. This type of polymerization is called slurry polymerization or suspension polymerization. The slurry solution is passed to a catalyst decomposition bed where the catalyst is deactivated. The catalyst is not completely used in the polymerization process. Catalyst decomposition is achieved with the addition of an alcohol. Polyethylene is then recovered with the extraction of the solvent, and a filtration and drying process. The polyethylene can then be processed and manufactured. The polyethylene created by the Ziegler process has a molecular weight 20,000 and 1.5 million. The molecular weight is controlled in a number of different ways: pressure of the reactor vessel (higher pressure, less branches), temperature in preparation of catalyst (too high of temperature deactivates catalyst), chain transfer reagents, and the ratio of Al/Ti catalyst added to reactor. The table below shows the weight average for different Al/Ti ratios for the Ziegler process at atmospheric pressure:
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Over the past decade, a number of synthetic routes for producing HAp powders have been developed
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polyethylene. Low Density polyethylene was being produced at extremely high pressures. This high-pressure polymerization created polyethylene with many branches; the branches are created due to intermolecular and intramolecular chain transfer during polymerization. The mechanism for the polymerization of low-density polyethylene is free radical polymerization. The uses of low-density polyethylene are limited due to high number of branches. Because of the extreme pressure needed to create low density polyethylene and its limited uses, Karl Ziegler was trying to create polyethylene at atmospheric pressure. Karl Ziegler, a German scientist, made the greatest contribution to producing high-density polyethylene.
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The difference in high-density polyethylene and low-density polyethylene is the degree of branching. The mechanical properties change drastically when comparing high-density polyethylene to low density polyethylene. In general, the degree of branching in polyethylene determines its mechanical properties. For example, high-density polyethylene is more crystalline than low-density polyethylene because it contains fewer branches. Unlike low-density polyethylene, Karl Ziegler created polyethylene with the use of a catalyst at atmospheric pressure. However, at first, this process did not go very smoothly. At first, Ziegler reacted Aluminum triethyl, a metal alkyl, with ethylene gas at atmospheric pressure. The reaction only yielded polyethylene with a molecular weight of 4,000. The reason for this is due to the displacement reaction where the aluminum-carbon bond displaces into a double bond.
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The reaction most important to controlling the molecular weight of high-density polyethylene created by the Ziegler Process is chain transfer with the introduction of hydrogen.
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