Polyethylene
Polyethylene (PE) is one of the most commonly used polymers which can be identified into two plastic identification codes: 2 for high-density polyethylene (HDPE) and 4 for low density polyethylene (LDPE). Polyethylene is sometimes called polyethene or polythene and is produced by an addition polymerisation reaction. The chemical formula for polyethylene is –(CH2-CH2)n– for both HDPE and LDPE. The formation of the polyethylene chain is created with the monomer ethylene (CH2=CH2).
The synthesis of polymers starts with ethylene, (or ethene). Ethylene is obtained as a by-product of petrol refining from crude oil or by dehydration of ethanol. Ethylene molecules compose of two methylene units (CH2) linked together by a double carbon
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bond. Throughout the polymerisation reaction, the double bond breaks and the extra bond attaches to another ethylene molecule repeatedly into a large polymer chain. These repeated units can be produced in linear or branched forms which are distinguishable between HDPE and LDPE. HDPE consists of very long hydrocarbon chains with no branches allowing the polymer to pack together to create a crystalline structure. This crystalline structure gives strength and stiffness to the polymer and retains its degree of flexibility. On the other hand, LDPE consists of more highly branched chains unlike HDPE and consequently does not adopt the crystalline structure. This gives the softer, weaker, less dense and more easily deformed properties of LDPE. The most common use of HDPE are in appliance housings, toys, bottles and some electrical insulation. All of these items are required to have the strength and stiffness in order to be fully functional of their uses. The most common use of LDPE is in products such as plastic bags, packaging film wrap, toys, some squeeze bottles as well as other electrical insulation material. Some disadvantages to using LDPE are that they have low strength, low stiffness, low maximum operating temperature and poor UV resistance. HDPE is quite similar except that its strength and stiffness is higher than LDPE. Main disadvantages to using polyethylene are the issues and impacts it has on the environment. Without special treatment to polyethylene, it isn’t very biodegradable and accumulates in the environment increasing the risk of animals consuming the polyethylene waste. Polytetrafluoroethylene Polytetrafluoroethylene (PTFE) is a polymer that is known by its trade mark name Teflon. The chemical formula for polytetrafluoroethylene is –(CF2-CF2)n–, which can be seen in Figure # produced from the addition polymerisation reaction of its monomer: tetrafluoroethylene (C2F4). The monomer tetrafluoroethylene is a colourless, odourless gas which is produced by heating chlorodifluoromethane (CHClF2) at temperatures around 600 to 700 degrees Celsius. These monomers are either suspended or emulsified in water to be polymerised under high pressure in the presence of free radical initiators. The final polymer product, Teflon, consists of a carbon chain with two fluorine atoms bonded to each carbon. These fluorine atoms bonded to the chain act as a protective layer with strong carbon–fluorine bonds. As a result, the polymer is chemically inert and has the lowest coefficient of friction of any known solid. This gives Teflon the non-stick, high chemical resistance and electrical insulation properties suitable for the most common uses of non-stick cookware, as well as being fabricated into industrial products including bearings, valve and pump parts. In 2005, reports of a harmful carcinogenic chemical known as perfluorooctanoic acid (PFOA) were used to make fluoropolymers. However, the company making Teflon and non-stick products claims that PFOA was used in the manufacturing process, however there were no PFOA left in the non-stick surface in the final product. When Teflon deteriorates at temperatures above the melting point (approximately 327 degrees Celsius), fumes which are omitted can be hazardous. These toxic fumes may include trifluoroacetic acid (TFA) and phosgene which are known to cause headaches, chills, backache and fevers – a condition known as the ‘Teflon flu’. However, in a normal household, there’s an extremely small chance of reaching 327 degrees Celsius while using non-stick products with Teflon to cook food. Therefore, there aren’t many disadvantages to the use of Teflon. Polyvinyl chloride Polyvinyl chloride (PVC) has a chemical formula of –(CH2-CHCl)n– created from the addition polymerisation of its monomer: vinyl chloride (CH2=CHCl), also known as chloroethylene. To produce vinyl chloride for the making of PVC, ethylene is heated to 150 degrees Celsius with chlorine and oxygen over a copper chloride catalyst. Free-radical initiators are highly reactive compounds that are introduced to vinyl chloride which splits the double carbon bond and links to other vinyl chloride monomers. This creates the repeating units of the polyvinyl chloride polymer. There are several structures or isomers that a PVC polymer can result in. The isotactic PVC is a polymer chain with a carbon backbone containing all the chlorine atoms on the same side as shown in Figure #. .
This maximises the intermolecular forces between the chains as they are able to pack closely together. However, this reduces the flexibility and is therefore quite rigid and strong. This isotactic polyvinyl chloride structure occurs very little. Another structure that is produced very little is the syndiotactic PVC. The syndiotactic PVC has a regular arrangement and has similar properties of flexibility, rigidity and strength to the isotactic PVC as shown in Figure #.
The most commonly produced PVC structure by addition polymerisation is the atactic PVC. As seen in Figure #, the chlorine atoms are branched randomly and asymmetrically along the carbon backbone. Unlike the other two structures, the random orientation prevents the polymers from packing closely together and is described to be ‘amorphous’.
Despite the orientation, atactic PVC is quite rigid due to the more electronegative chlorine atoms. This also provides the strength and rigidity for suitable uses in pipes, window and door frames and even credit cards.
PVC is classified as a thermoplastic, which can be re-shaped in the presence of heat. Situations where temperatures reach up to 160 degrees Celsius (melting point) would cause deformation to the PVC; this would be considered a
disadvantage. Polyethylene terephthalate Polyethylene terephthalate (PET, PETE) is a polymer often used in plastic bottles. The chemical formula for PET is –(C10H8O4)n– compiled by monomers of terephthalic acid (C8H6O4) and ethylene glycol or 1,2-ethanediol (C2H6O2) by condensation polymerisation as shown in Figure #. . A type of condensation polymerisation is when a carboxylic acid is reacted with an alcohol to produce an ester and water, which is known as esterification. Whilst being heated with chemical catalysts, PET is produced as a molten, viscous mass which is either spun directly into fibres or solidified to be processed later. The PET units are adjoined together by ester groups formed from the reaction of the hydroxyl and carboxyl groups. Terephthalic acid contains a large aromatic ring and repeated in the polymer to provide the stiffness and strength properties. Nylon Nylon 6,6 is synthesised by the condensation polymerisation reaction of 1,6-dihexanoic acid (adipic acid) and 1,6-diaminohexane. Another type of condensation reaction is known as amidification. Amidification consists of a carboxylic acid reacted with an amine molecule to produced an amide and water.
Have you ever wondered why the plastic bag that you left on the porch during winter cracks or breaks more easily than when you left it during summer time but a piece of wood which was left just like the plastic bag has no effect whatsoever? This is because of a phenomenon, which only happens to polymers, known as the glass transition. For each polymer, there is a certain temperature at which the amorphous polymers undergo a second order phase transition from a rubbery and viscous amorphous solid to a brittle and glassy amorphous solid called the glass transition temperature, Tg.1 When the polymer, or in this case, the plastic bag, is cooled below their glass transition temperature, it becomes hard and brittle like a glass but when it is used above their glass transition temperatures, it might have a different effect than when used at room temperature or below the glass transition temperature as normally, different types of polymers like clothes, food packaging, insulations for wires, etc. are either used above their glass transition temperatures or
Before using ethylene to produce polyethylene, the compound needs to be purified to almost 100%. In order to reach this level of purity the ethylene needs to be freed of olefins, acetylenes, dienes and water through several processes such as: driers are used to take out the water, a demethanizer is used to remove methane, etc. ...
A group of polymer chains can be organised together in a fiber. How the polymer chains are put together is important, as it improves the properties of the material. The flexibility, strength and stiffness of Kevlar fiber, is dependent on the orientation of the polymer chains. Kevlar fiber is an arrangement of molecules, orientated parallel to each other. This orderly, untangled arrangement of molecules is described as a “Crystalline Structure”. A manufacturing process known as ‘Spinning’ is needed to achieve this Crystallinity structure. Spinning is a process that involves forcing the liquefied polymer solution through a ‘die’ (small holes).
Polymer creatures are very fascinating, because they can grow when put in liquids, and experiments are going to be conducted relating to this growing. A polymer is a very long chain of molecules strung together (What). Polymers are very versatile, and can have almost limitless colors and characteristics (Definition). One of these qualities is absorbency, and some can absorb as much as five hundred times their weight in water (Growing). It is hypothesized that if the polymer creatures are submerged in water for a day it will at least double in size, while it will not grow as much when put in Sprite.
The plastic obtained from polyethylene can be pressed into varying shapes, ranging from the simplest to the most complicated. The ethylene market is indirectly driven by the increased polyethylene consumption for the production of several plastic components. In 2011, the global propylene market is valued at more than $ 90 billion and significant growth is anticipated in the coming years. Acrylic fibers and coatings, PVC plasticizers and coatings, polyurethane resins, epoxy resins and propylene polymers are applied on polycarbonates and solvents, that are used in the production of propylene. The automotive industry is the biggest end-user of polypropylene. The enormous expansion of ethylene and propylene production will contribute to the growth of the ethylene and propylene market, along with the rising demand for downstream products from India, China, and the Middle East. Tight environmental regulations, fluctuations in raw material prices and political uncertainties in crude oil producing regions are the main factors limiting the ethylene and propylene market [9].
Clive Maier, Theresa Calafut. 1998. ‘Polypropylene: The Definitive User's Guide and Databook’. USA: William Andrew Inc.
chains instead of hydrogen atoms. Cross-linking is another way in which the polymer can be made stronger. This involves ultraviolet radiation that bombards the polymer with electrons and formulates bonds between the molecular chains of the polymers. This is like linear polyethylene but different in that it is more impact resistant, and it has a much higher density. This allows it to be stored or be used with different chemicals that would normally cause the polymer to desolve.3 This can start to become a problem because as the polymer continues to become chemically enhanced. So the ways of dissolving and recycling the polymer become more difficult.
Monomers that are combined by condensation polymerization have two functional groups. A carboxylic acid and an amine can form an amide linkage, and a carboxylic acid and an alcohol can construct an ester linkage. So as each monomer has two reactive sites, they can form long chain polymers by making multiple amide or ester links and releasing a small molecule. Examples of the mentioned mechanism are illustrated below.
While highly branched PEP result in rigid PU with good heat and chemical resistance, less branched PEP gives PU with good flexibility and low chemical resistance.
Determines crystalline to amorphous transition temperatures in polymers and plastics and the energy associated with the
Polymers are made from relatively small molecular fragments known as monomers that are joined together. Synthetic polymers which include the large group known as plastics are divided into three groups: commodity thermoplastic, engineering thermoplastics (ETP), and advanced engineering thermoplastics (AETP). The engineering thermoplastics (ETP) have heat resistance, strong mechanical properties, lightness, self-lubrication, and easy manufacturing. This plastic category has been lately used to replace wood and metal applications.
Ever since I began studying science and mathematics at all levels of educations I have always had an interest in the production of useful materials. In the growing turmoil of today; a world full of global warming and diminishing resources, questions often arise in my mind such as, "can we make a more efficient, more durable and a renewable resource that will overshadow fossil fuels? and have less of an impact on our environment?" Up to now, I have not found a solution to these questions and answering these questions is a personal aspiration of mine which I aim to fulfil by achieving a degree in Chemical engineering and eventually I will contribute to the field in my own unique way. The debate surrounding sustainable energy fascinates me, having recently learned from personal research I have understood what an authoritative role chemists and chemical engineers play in the industry at the present time and how, by working as a team, they contribute to an improved future for the whole world. However, one of the main reasons that has single-mindedly driven me this far to want to study chemical engineering is a book I have read, “Beyond the Molecular Frontier: Challenges for Chemistry and Chemical Engineering” While reading this book, I had solidified my understandings of what chemical engineering is all about. Also, one of the main processes mentioned was polymerisation and is something I already study in A-level chemistry, it is something that not only interests me, but is a personal career aspiration of mine. Reading this book gave me a determination to be the person who helps improve the future of the industry and provide an answer to the questions I always ask myself by studying this degree.
Therefore, plastic packages are coded to indicate the type of resin used to make them. The
Polymer means any of various chemical compounds made of smaller identical molecules called monomers linked together. Some polymers, like cellulose, occur naturally. Polymers have extremely high molecular weights, and made-up of many of the tissues of organisms, and have various uses in industries. The process by which molecules are linked together to form polymers is called polymerization (The American Heritage Science Dictionary, 2005). Polymeric compound is a compound made of many smaller molecules such as cellulose, chitin, soy protein, casein and many more. Polymeric is an organic giant molecule and most of the compound is non-crystalline.
First, understanding the basics of polymers. Polymers are very large molecules formed by combining many smaller molecules together chemically. The starting molecules of polymers is called monomer which on combination forms random co-polymers. Co-polymers like SBR are widely used as a modifier in asphalt mix. They are very widely used for preventive pavement maintenance technique in cold weather like in chip seals, slurry seal, cold recycled mix and micro-cracking.