Free «Polymers» Essay
In chemical terms polyethylene is a polymer from ethylene (ethene) (Allcock et al. 2003). Polyethylene was first obtained by the German scientist Hans von Pehmanom in 1898 (Allcock et al. 2003). As it often happens in chemistry, its discovery occurred by an accident when Pehman warmed diazomethane (Allcock et al. 2003). His colleagues – Eugen Bamberger and Friedrich Chirner – characterized the resulting material as a white, waxy substance (Allcock et al. 2003). Finding that it included long chains of-CH2-, they called it a polimetilenom material (which may be more accurate in terms of the properties of the double bond between carbon atoms) (Asakara et al. 2003). The synthesis of polyethylene suitable for industrial use was also associated with a chance discovery in 1933 (Asakara et al. 2003). This time the British discovery was made by Eric Fawcett and Reginald Gibson, the employees of Imperial Chemical Industries (ICI) (Asakara et al. 2003). Polyethylene at this time was formed by mixing ethylene and benzoic aldehyde. Firstly, it was not possible to repeat the action because it was initiated by present oxygen impurity in the apparatus (Asakara et al. 2003). However, this was achieved in 1935 by another ICI chemist, Michael Perrin, who created a technology that was the basis of industrial production since the beginning of LDPE in 1939 (Asakara et al. 2003). Further improvement of the technology took place mainly through the introduction of new catalysts, enabling production of higher quality materials (Asakara et al. 2003).
Polyethylene is a thermoplastic synthetic nonpolar polymer belonging to the class of polyolefin. This is a product of ethylene polymerization. It is white and solid. It comes in the form of high-density polyethylene (HDPE) obtained by suspension polymerization of ethylene using a low pressure on complex organometallic catalysts in a slurry or gas phase of ethylene polymerization using a gas phase on complex organometallic catalysts, and high-density polyethylene (LDPE) produced at high pressure ethylene polymerization in a tubular reactor or reactors with mixing device using a radical type initiator (Wright 2001). In addition, there are several subclasses of polyethylene, different from traditional higher performance. In particular, there are high molecular polyethylene, linear low density polyethylene, polyethylene obtained by metallocene catalysts, and bimodal polyethylene (Wright 2001). Typically, polyethylene is produced in the form of stabilized pellets 2.5 mm in diameter in the painted and unpainted form. However, there is commercial production of polyethylene in a powder form (Wright 2001).
Usual notation of polyethylene is PE, but other designations can occur: PE (polyethylene), LDPE or HDPE (low density polyethylene, high-density polyethylene), or MDPE or PEMD (medium density polyethylene), ULDPE (ultra low density polyethylene), VLDPE (very low density polyethylene), LLDPE or PELLD, LMDPE (average density), HMWPE or PEHMW (high molecular weight polyethylene). HMWHDPE (high-molecular polyethylene density), etc (Crivello et al. 2004).
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Polyethylene is a plastic material with good dielectric properties. It is shock-resistant; it does not break and has little absorptive capacity. It is physiologically neutral, and odorless. It has a low vapor and gas permeability (Crivello et al. 2004). Polyethylene does not react with bases at any concentration, with solutions of any salts, carbon, concentrated hydrochloric and hydrofluoric acids. It is resistant to alcohol, gasoline, water, vegetable juices, and oils. It can be destroyed with 50% nitric acid, and with liquid and gaseous chlorine and fluorine (Crivello et al. 2004). It is not soluble in organic solvents and is limitedly swelling in them. Polyethylene is resistant when heated in a vacuum and inert gas atmosphere. However, in the air it is degraded by heating, even at 80°C (Crivello et al. 2004). What is more, it is resistant to low temperatures down to -70°C (Crivello et al. 2004). Under the influence of solar radiation, especially ultraviolet rays, it is subjected to photo destruction. Overall, it is practically harmless; it does not produce environment hazardous substances to human health (Mariani et al. 2003).
Polyethylene can be easily processed by all major plastic processing methods. It is susceptible to modification. Through chlorination, sulfonation, bromination, and fluorination it can acquire the rubber-like properties, heat resistance and chemical resistance are improved (Ezrin 1996). Copolymerization with other olefins and polar monomers increase the resistance to cracking, flexibility, transparency, and adhesion characteristics. Mixing with other polymers or copolymers improves the strength and other physical properties (Ezrin 1996).
Chemical, physical and performance properties of polyethylene depend on the density and molecular weight of a polymer, and therefore, they are different for different types of polyethylene. For example, low-density polyethylene (PE with branched chain) is softer than HDPE, so films of HDPE are tighter and denser than those from high-density polyethylene (Fiori et al. 2004). Their tensile strength and compressive strength has greater tear resistance, and permeability is 5-6 times lower than that of LDPE films. UHMWPE with a molecular mass of more than 1 million has a higher strength quality. Its operating temperature ranges from -260 to +120°C. It has a low coefficient of friction, high wear resistance, crack resistance, and chemical resistance in the most hostile environments (Mariani et al. 2004).
Polyethylene is the most widely used polymer. It leads the global release of polymeric materials – 31.5% of the total volume of produced polymers. Technology for manufacturing products made of polyethylene is relatively simple. It may be subjected to processing with all known methods. It is welded by major ways: hot gas filler stick and friction welding. Work with polythene does not require the use of highly specialized equipment, such as for PVC processing, and modern industry produces hundreds of brands of additives and dyes to make products of polyethylene with a variety of consumer qualities (Odian 2004).
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Using injection molding, a wide range of household goods, stationery, and toys is produced from polyethylene. When using extrusion plastic pipe are produced (there are special marks – Pipe PE63, PE80, PE100), as well as polyethylene cables, polyethylene sheet for packaging and construction, as well as a wide variety of plastic films for the needs of all industries (Odian 2004). Extrusion blow molding and rotational molding of polyethylene benefits to creation of a different kind of containers, vessels, etc. Various special types of polyethylene, such as cross-linked, expanded, chlorosulfonated, superhigh, are successfully applied to the creation of special materials. Separate segment of the market is recycled polyethylene (Odian 2004). Many companies in the world specialize in buying plastic waste with the further processing and sale or use of recycled polyethylene. Generally, this applies to the technology of extrusion of treated waste and the subsequent splitting and getting recycled granular material suitable for the manufacture of products (Odian 2004).
Most widely polyethylene is used for the production of films for technical services. Benefits of all types of polythene for packaging purposes are as following: low density, good chemical resistance, little water absorption, good transparency, easy processability, good weldability, impermeability to water vapor, high strength, flexibility, extensibility and flexibility (Gross et al. 2003). Plastic films are used to make bags for bread, vegetables, meat, poultry, and garbage. Thanks to the heating and adhesion with other materials, they are also suitable for the use on the carton and other packaging materials. Domestic ethylene with vinyl acetate is produced by polymerization of ethylene and vinyl acetate in the mass under high pressure. It is widely used in the production of twisted hose equipment (Kiryukhin et al. 2003).
What is more, polyethylene is used to make:
- films: agricultural, packaging, shrink, stretch;
- tubes: gas, water, pressure, non-pressure;
- tanks: cans, bottles;
- building materials;
- household items;
- sanitary wares;
- parts of vehicles and other equipment;
- electrical cable insulation;
- prosthesis of internal organs (Gross et al. 2003).
In fact, this is not the limit of the possibilities of using polyethylene. Moreover, the market is constantly facing new brands of polymer with new consumer properties. For example, UHMWPE is used for the manufacture of high-technology products that are resistant to impact, cracking and abrasion: gears, bushings, couplings, pulleys, rollers, sprockets, and insulating parts of equipment operating in the range of high and ultra-high frequencies (Gross et al. 2003). In addition, the UHMWPE is widely used in the manufacture of porous products: filters, silencers, gaskets, and prosthetics – to create joints, cranial and maxillofacial implants.
In the 20th century, humanity has gone through a synthetic revolution. Its main achievement can be called the invention of plastic. It is difficult to imagine that in the beginning of the last century, it did not exist, and everything was done from fashionable natural materials.
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It may be said that mankind played a game before the invention of plastic. In the history of this material a mystical relationship with the love of people to play with a ball can be traced. In the 2nd century BC, the Greeks played a ball made from a pig’s gallbladder, filled with air. This sports equipment was shaped like an egg or a rugby ball. Even then, our ancestors sought to correct the shape of a ball and make it perfectly round. Greeks endlessly tried various herbal supplements to give a pig’s bladder elasticity (Dzhardimalieva et al. 2003).
The Mayan ball was made from the peel of fruit, wrapped in natural rubber, which they obtained from figs. Similar technology was used by the Islanders of Oceania and Southeast Asia. However, it was well-developed only by Europeans (Lewis 2004). In the 19th century, gutta-percha tree was brought from Malaysia to Europe from which latex was extracted (Commissiong et al. 2003). The first products from the new material were golf balls. Today this material is used to insulate submarines, underground cables and adhesives (Lewis 2010).
The ball was passed to billiards. In 1862, a British chemist Alexander Parkes decided to invent a cheap substitute for expensive ivory used to make billiard balls (Commissiong et al. 2003). The result was the opening of the first plasticizer. Parkes invented the first nitrocellulose. However, its properties were not suitable for a playing ball, because the material was easy to crash. There was a need for a supplement that would alleviate it without reducing the main useful properties – elasticity (Commissiong et al. 2003). Parkes decided to add camphor. A mixture of nitrocellulose, camphor and alcohol was warmed to a fluid state, then poured into a mold and solidified at normal atmospheric pressure. Thus, the first semi-synthetic plastic was born. Alas, a pioneer did not achieve a commercial success.
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Still, a follower of Parkes, an American John Hite earned the first plastic fortune (Campbell 2000). He founded the company and began to make combs, toys and lots of other products from celluloid. Unfortunately, the material was highly inflammable so now only table tennis balls and school rulers are made from it (Campbell 2000).
However, plastic, in addition to all of its remarkable properties, has two major drawbacks. Firstly, it is made from non-renewable natural resources – oil, coal and gas. Secondly, its main advantage – durability – which was chased by the inventors of plastic in the beginning of the last century today turned into a disadvantage (Campbell 2000). The more plastic we use, the faster the mountains of waste which does not degrade in the environment under any circumstances grow (Lewis 2004). Millions of tons of plastic are collected in nature and pollute the environment.
Therefore, closer to the end of the last century, scientists have thought about how to create a material that is similar to the properties of plastic. Meanwhile, it was required to substitute it with plastic which can be made from renewable components (e.g. plants) and which would not be too tough for the bacteria to decompose in nature. In the mid-1990s, sensational reports about the invention of bioplastics – plastics from a natural starch degraded by exposure to various microorganisms – began to appear (Pojman et al. 2003). However, then, a large-scale innovation in our daily life was out of the question: the production of bioplastics was too expensive (Pojman et al. 2003).
In the new century, the situation changed dramatically. Scientists have found a way to reduce the cost of bioplastics manufacturing and claimed that it would soon be close to the cost of manufacturing conventional plastics (Pojman et al. 2003). In fact, some experts believe that the price of degradable plastics was artificially inflated by commercial manufacturers and oil companies (oil companies do not favor bioplastics because its mass production can lead to a drop in oil prices) (Pojman et al. 2003). However, if to count the cost of recycling plastic waste and make this figure in the cost of conventional plastic, it is not clear which one will be more expensive (Pojman et al. 2003).
Conventional plastic is not degradable in the environment, because it consists of very long polymers, which are closely related to each other. Plastic, containing natural shorter polymers of plants, behaves quite differently. Bioplastics can be made from the starch, which is a natural polymer and is produced by plants during photosynthesis. Starch is abundantly present in grains, potatoes and other undemanding plants. Harvest of corn starch counts up to 80% of the collected green mass. Therefore, the production of a new generation of plastic should be sufficiently profitable (Ritter et al. 2003). Bioplastics break and crumble at any temperature in which there is an activity of microorganisms. Residual products of the process are carbon dioxide and water.
Since starch is easily soluble in water, products from it are easily deformed by the slightest contact with moisture. In order to give greater strength to starch, it is treated with specific bacteria, which biodegrade polymer starch into lactic acid monomers (Ritter et al. 2003). Then, monomers chemically are chained into polymers. These polymers are much stronger, but not so long as the polymers of plastics, and can be biodegradable. The resulting material is called polylactide (PLA). Last year the world's first plant for the production of PLA was opened in Nebraska (Sperling 2006).
Another way to obtain bioplastics is to use bacteria Alcaligenes eutrophus (Sperling 2006). In the course of their life they produce organic plastic pellets. Already successful experiments were carried out to introduce genes into the chromosomes of the bacteria of plants, so that they could continue to make plastic inside their own cells (Sperling 2006). This means that the plastic can be literally grown. However, such a method is still expensive. Moreover, since the process involves intervention at the genetic level, it has its opponents.
About 30% of non-degradable, inert plastics is used for packaging and discarded immediately after opening a he package (Workman 2001). Plastic waste counts for 15% of municipal waste, an average of 20 kg per person per year (Workman 2001). Given that the annual increase in the production and use of plastics in the last decade is reaching 10-12%, it is clear that the problem of recycling becomes more urgent with every year (Workman 2001). The idea that it would be very good if the polymers themselves under certain conditions in a given period are decomposed into harmless to living and non-living components of nature is perfectly logical. Technology of polymers decomposing when exposed to sunlight, water, temperature, and microorganisms that have received an appropriate name – photolysis, dissolution, hydrolysis, thermal and biodegradation – is already developed (Workman 2001).
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However, except for the biodegradation, these processes do not give satisfactory results since just like burning they remove only the visual manifestation of pollution. They are also difficult to regulate. To date, the biological degradation of polymer is the first direction after recycling plastics waste. Biodegradation sometimes is understood as the amount of microbial processes, resulting in the mineralization of organic elements and it also includes the loss of certain physical properties, appearance of fragility and friability, disintegration of polymer. The current generation of people finally became convinced that our environment – land, water and air – cannot have an infinite immunity to chemical use. Although there is a carefree attitude to nature, people have begun to understand and re-evaluate the catastrophic consequences. Therefore, polyethylene presents a burning issue which should be solved in the nearest future.
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