The ultra-high molecular weight polyethylene was first attempted in 1942 which involved the formation of polyesters that came from Carothers und Hill and the aliphatic diols. This fuelled the production of the PBT fibers and later there was a market launch of the same. This polymer is a higher quality polyethylene that has a number of synthesis, applications, yearly production, and processing that will be intensively be discussed in the paper. What makes the ultra-high molecular weight polyethylene to have unique thermal, mechanical, electrical and optical properties is due to its ultra-high molecular weight. The difference in weight with other molecular substances makes the material to be stronger than the rest. Such strength enables the compound to withstand any form of abrasion than the other lower level poly compounds. The number of the processing methods that are discussed in the paper all depends on the molten state of the liquid as it does have a higher molecular weight. Later in the paper, it will be justified enough that the ultra-high molecular weight polyethylene cannot be molded or transformed by injection molding, thermoforming or other conventional plastic processing techniques.
According to Huang et al. (2016), ultra-high molecular weight polyethylene has a molecular weight numbering and also belongs to the class of linear polyethylene which is also well known as the high-performance PE or the high modulus PE. The synthesis of the ultra-high molecular weight polyethylene consists of magnesium chloride that works well in conjunction with the Ti-based catalyst that is often abbreviated by Cat A-D. For the preparation to be successful, the experiment must consist of triethylaluminium which has to be a solution of 0.5M contained in an n-hexane. In addition, there has to be an external donor, for example, the propyl-i-propyl dimethoxy silane (PiPDMS), Phenyltriethoxysilane (PTES) and di-n-propyl dimethoxy silane (DnPDMS). The n-hexane to be used has to be first be distilled under nitrogen from benzophenone or simply sodium. The whole process of polymerization is often carried in a 300 ml glass reactor that has to be accompanied with a magnetic bar to stir the solution.
During the process, nitrogen is used to fill in the reactor for more than three times while a considerable amount of n-hexane is used to charge the nitrogen in the reactor. The solution later is vigorously stirred which has to be maintained at a temperature of 40oC and also at an atmospheric pressure of 1 atm. Later still under the above conditions, the cocatalyst is usually added to the reactor. Immediately the cocatalyst is added, the catalyst solution is injected together with the external donor where the whole process of polymerization starts in a continuous manner as the ethylene is fed (Huang et al., 2016). One point to note is that the temperature has to be maintained all through the whole process with the help of a water bath. After a period of 2 hours, the polymerization process is cut off simply by introducing hydrochloric- methanol solution that is 10 percent and later transferring the mixture into a methanol solution of 500 ml. The moment this is done, immediately the polymer becomes precipitated where drying has to be done later. A vacuum under a temperature of 60oC is used at the same time ensuring there is a considerable weight altogether. When all this is done, finally the person doing the experiment ends up with the ultra-high molecular weight polyethylene.
According to Huang et al. (2016), the second method to prepare ultra-high molecular polyethylene is through the addition of nanocrystalline silicon carbide to a certainly given solvent. The two are repeatedly ground by the help of a sand mill where later they are mixed to form a uniform mixture. A spinning solution is also gotten from a high shear with a homogenizer and later subjecting the mixture to a conventional gelatin spinning. The gel filament is drawn and extracted where a composite fiber is extracted. This method has been proven to come up with a high resistive polymer in terms of mechanical and chemical properties. On the other hand, a considerable ultrafine micro powder that can be used include either silicon, boron, carbide, oxide or even a combination of the two. What is important when two substances are combined is that a diameter of 0.1 to 300 micrometer have to be maintained and also the whole content has to be in between 0.1 to 14 percent of the total weight of the fiber.
The third method to prepare ultra-high molecular polyethylene is through the use of a slurry. This is totally different from the mode that has normally been used where gas polymerization has often been used. With the use of this kind of method, the polyethylene which at the end is produced is established to have a higher crystallinity and molecular weight both resulting from a conventional process (Huang et al., 2016) However, this involves the use of the same catalyst formulation. At last, a polyethylene with a crystallinity of 50 percent and a molecular weight of 200 to 300,000 g/mol is obtained all this taking place in the absence of the unreactive compound. The post reaction high pressure becomes easier when the need arises to increase the crystallinity of the polymer. This has also been proved to be cheaper due to the less energy input. The crystallinity improvement is facilitated by the immense pressure coming from the unreactive compound as ethylene is being polymerized. The technical effect being experienced in the course of the process comes from various polymerization processes such as the crystallinity that has to be obtained only in the presence of the unreactive compound.
Ultra High Molecular Weight Polyethylene Powder, UHMWPE powder (2017)
From the above picture, the enhancement and easiness to improve crystallinity of the polymer come from the high molecular weight chains which often leads to having a more ordered structure. The unreactive compounds that are often encouraged to be used in the process are the inert compounds as they contribute to less production of crystals. The kind of ultra-high molecular polyethylene produced is in most cases of 35 percent crystallinity. However, there are possible improvements in crystallinity of 5 to 10 percent if the synthesis of the whole process is carried out in the absence of the inert compounds that are unreactive. Not necessarily that this kind of method to prepare ultra-high molecular polyethylene is restricted to use only the inert compounds as the only unreactive compounds but also unreactive compounds of the isomers of alkanes may also be used. The unreactive compound is often injected into the reactor before or after the process of polymerization.
During synthesis, if the unreactive compound is a liquid, then it can be injected and then later be vaporized. The molar ratio of the unreactive compound is said to vary from 0.001 to 30. For instance, if the unreactive compound being used is the C6 alkane, then it can be 0.12 or less. The vapor phase of such kind of processes only requires the optional olefin comonomer, ethylene, and the unreactive compound. The whole ethylene polymerization usually takes place under temperatures ranging from 10 to 150oC (Huang et al., 2016). Olefin polymerization is only supported when the polymerization process takes place in the gas phase reactor. However, not necessarily that this particular catalyst is used where other types catalyst more of conventional Ziegler-Natta catalysts can be used. Other examples of catalysts that may be used include the titanium/magnesium Ziegler-Natta catalyst, supported metallocene-type catalyst and Phillips-type catalysts such as chromium oxide contained in silica.
According to Hyon & Oka (2014), the mechanical properties of the ultra-high molecular weight polyethylene consists of tensile strength at yield which is 21 Mpa, tensile strength at break is 48 Mpa while elongation at break is 350 percent. In addition, we have the Young's Modulus were at 23 centigrade it is said to be 0.69 while at -40 centigrade the modulus is 0.69. The Izod impact strength of the ultra-high molecular weight polyethylene is 0.691 kJ/m at 23 centigrade while at -40 centigrade is 1.1 kJ/m. Still, under the mechanical properties, we have the hardness and shore which has been discovered to vary from 62-66 kJ/m. In addition, we have the abrasion resistance that has been proved to be 100. Finally is the relative solution viscosity which ranges from 2.3-3.5 dL/g. The thermal properties of the ultra-high molecular weight polyethylene include the crystalline melting range which lies between 138 -142 F and the coefficient of linear expansion in between 20 to -100 Celsius is 2 while that of between -200 to -100 has been proved to be 0.5 which all results from the structure of the polymer.
UHMWPE: Lightweight & Abrasion Resistant (2017)
The units that are often used to illustrate the thermal properties of ultra-high molecular weight polyethylene are either Celsius or Kelvin. When we look at the electrical properties of ultra-high molecular weight polyethylene, it includes the volume resistivity, dielectric strength, dielectric constant, dissipation factor and surface resistivity. Its volume resistivity is 5x104 ohm-cm, the dielectric strength is 2.7, the dielectric constant is 0.001 while the dissipation factor depends on the types of hertz used. Finally, the surface resistivity of ultra-high molecular weight polyethylene also depends on the surface on which it is exerted to, color, protection, antistatic applications as well as the conductive applications (Hyon & Oka, 2014). Some of the optical properties of the ultra-high molecular weight polyethylene include the reference which has to do with a light transmission having no aspect of interaction which is 9.6 percent.
This aspect of light transmission has to do with the culminated transmitted light that is usually 10 percent. However, most of their reference ends out to be absorbed in the same time4 percent being reflected back from the polyethylene surface. The goodness of the absorbance curve has a considerable value which is greater than 98 percent of which in most cases, it has the following absorption peaks: 632nm, 532nm, 500nm, 460nm, and 390nm. Both the 460 and the 390nm usually belongs to the strong absorption which does take place on the surface. This does happen because of the lower penetration depth at both the peaks and the wavelengths. According to Hyon & Oka (2014), the scattering of the ultra-high molecular weight polyethylene is always total forward directed that helps to justify the antistrophic nature of the polymer.
The ultra-high molecular weight polyethylene has a variety of applications. This makes it be one of the most useful and distinctive material. Due to the excellent properties that the polymer has, this makes it have outstanding superior impact resistance, excellent mechanical properties, considerable abrasion resistance and the other self-lubricating properties (Hyon & Oka, 2014). The above-named properties drive many engineers to prefer the use of the polymer in the following sectors: the polymer is used in the chemical, mineral and mining processing equipment as well as the orthopedic implants, in food beverage and machinery, in bulk material handling as well as the number of recreational equipment's available. This all applications are supported by the good frictional properties the ultra-high molecular weight polyethylene has, high chemical and abrasion resistance. In addition, we have the industrial applications that include gear housing, door control services, electronic stability programmers and the steering angle senders.
According to Hyon & Oka (2014), due to the polymer optical properties, the main use of the ultra-high molecular polyethylene evident in the application of window frame profiles as well as the fiber optical cables. The mechanical properties of the polymer facilitate some applications such as wipers and the headlights that are found in various automotive. Other applications of the polymer that has been there over the past few years include the suspension wear plates, chain guides, belt scrapers and the conveyor guide rails all of which are facilitated by the polymer's mechanical, optical, thermal and electrical properties as described in the picture below.
China Ultra High Molecular Weight Polyethylene (2017)
Since the polymer has a low coefficient of friction of which has been proved to be 0.14 this makes it more applicable and perform better in the steel sliding applications. This is also facilitated by its lubricity nature which tends to help in minimizing friction, have a noiseless operation, the whole operation is often smooth while at the end there is less damage to the steel parts being used.
The ultra-high molecular weight polyethylene also has the capability to withstand low temperatures, grit as well as sand. Furthermore, the ultra-high molecular weight polyethylene is said to have favorable chemical properties which enhance its application as well as of great help when it comes to snow sliding where the frame in a snowmobile drive is protected. This qualifies as a tough endurance test for any material as long as it is in conjunction with the polymer. Due to the aspect of wear resistance, low friction, and durability, this makes the ultra-high molecular weight polyethylene to be used in waste water treatment that involves wear shoes and strips, conveyor wears plates, bottling and canning processes and also the guide shoes for patilizers (Hyon & Oka, 2014). From the polymer having abrasive friction nature, it has often been used in the past few years in the high-speed drive chain. Furthermore, since the ultra-high molecular polyethylene has high resistance to wear and tear, extreme biological inertness and thermoplastic material combining light weight, this has made it be used to make bulletproof vests, machines parts, and medical prosthetics.
From the high demand attributed with the ultra-high molecular weight polyethylene, the production of the polymer has been relatively higher in every year. Simply year after another, the production of the polymer has been rising drastically. This has resulted to the newly discovered uses of the polymer by both scientists and the manufacturers. According to Hyon & Oka (2014), an amount of 200, 000 tons of UHMW products used in the automotive industry to enhance conveyor lines and also used in the beverage companies such as shown in the picture below demand has been rising day after another.
Garland Manufacturing Company - GarDur UHMW Polyethylene (2015)
In every year where almost 12,000 tons are being produced domestically in the countries that do manufacture ultra-high molecular weight polyethylene. The development potential of the same in the unsaturated markets has been considerably high.
To consolidate the powder into solid form, pressure, time and temperature are optimized (Hyon & Oka, 2014). The main four methods that are used to process the ultra-high molecular weight polyethylene plates as seen in the picture below include the ram extrusion, compression molding, hot isotactic pressing and the direct compression molding.
High-Density Polyethylene Sheet (XH869) (2017)
The intrusion method takes place in a cylindrical bar stock that mostly ranges from 2 to 6 inches diameter. The ram retracts and introduces the powder into the heated cylindrical barrel where later the whole chamber is filled with the powder. The powder has to be consolidated into a continuous bar due to the existing pressure and heat. Furthermore, the addition of CA stearate also takes place where this is regarded as important to the raw powder. Large sheets are later produced when compression molding does take place.
Compression molding entails introducing the ultra-high molecular polyethylene powered into a cavity, heating the same powder and finally compressing of the plate does take place. Sectioning and turning off the sheets into a lathe is important in the process too. The powder is heated and later compresses when direct molding is applied all this taking place in a mound. The sheets that are produced at the end are machined to make the final products of ultra-high molecular weight polyethylene.
In summation, ultra-high molecular weight polyethylene has been one of the major inventions in the past two decades where it has been appraised due to its variety uses. This has not only made life easier through the various application but also the manufacturing process has become easier since it was invented. All this has been demonstrated by the paper looking at preparation (synthesis), properties that include the mechanical, thermal, electrical and optical conditions. In addition, the polymer information has been expounded by having looked at the various application that has been emphasized by scientists mostly in the manufacturing sector. Furthermore, there is the yearly production of the ultra-high molecular weight polyethylene which has a relatively high demand both in the countries producing the polymer and those in need of it. Finally is the processing aspect where the polymer has to undergo a number of stages in coming up with the final articles. Processing has been comprehensively being discussed by having a look at the following main methods that are used to process the ultra-high molecular weight polyethylene: the ram extrusion, compression molding, hot isotactic pressing and the direct compression molding. Mostly the US government has encouraged the use of this polymer in the development of personal protective equipment that includes body armor and helmets. There also exist some application insights of the polymer for example in the construction, healthcare, sports equipment, sports, and defense industry. More increase demand in the healthcare industry, sports events, and construction spending will help to cater for overall future consumption of the polymer.
China Ultra High Molecular Weight Polyethylene. (2017).
Garland Manufacturing Company - GarDur UHMW Polyethylene. (2015).
High Density Polyethylene Sheet (XH869). (2017).
Huang, G., Ni, Z., Chen, G., & Zhao, Y. (2016). The Influence of Irradiation and Accelerated Aging on the Mechanical and Tribological Properties of the Graphene Oxide/Ultra-High-Molecular-Weight Polyethylene Nanocomposites. International Journal of Polymer Science, 2016.
Hyon, S. H., & Oka, M. (2014). U.S. Patent No. RE44,762. Washington, DC: U.S. Patent and Trademark Office.
UHMWPE: Lightweight & Abrasion Resistant. (2017).
Ultra High Molecular Weight Polyethylene Powder, UHMWPE powder. (2017).
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