The newly created free radical breaks the carbon-to-carbon bond, swiping an electron, and creating a new free radical with a single unpaired electron on the end.
This continues, as a chain reaction, with a long chain forming as more ethylene molecules are added. The process keeps going until free radicals meets another free radical, completing the chain. Now we have our polymer, polyethylene, made up of the monomer repeating unit ethylene.
Some other examples of polymers formed in this way are polychloroethylene PVC , used to make things like plumbing pipes and insulation for electrical cables, and polypropylene, used in products such as rubber ducks and other toys and, when processed into fibres, carpets.
The way molecules are arranged gives different polymers different properties. Polyethylene, for example, has long polymer chains side by side. When they cool down, the chains interact and become entangled. Polyethylene can be melted and reformed into a new shape over and over again. These meltable, reshapable polymers are known as thermoplastics. Other examples include polystyrene and polypropylene. The strength of polymers also varies depending on how the molecules are arranged.
To use our paperclip analogy, you may decide to have some paperclips branching off your main line. They result in a polymer with a lower density. Low-density polyethylene LDPE —the squishy material that plastic bags and wrap like the kind you might wrap your sandwich in —is an example. The resulting polymer is stronger and has a higher density. An example is high-density polyethylene HDPE , used to make things like plastic bottles, food containers and plumbing pipes.
In contrast to thermoplastic polymers are thermosetting polymers. It is useful, though, for things like car tyres, since a tyre that melts in the heat is going to make for a pretty interesting drive to the beach. Glues and electrical components are also thermosetting polymers. As well as the arrangement of molecules, the properties of a polymer are also determined by the length of the molecular chain.
In a nutshell, longer equals stronger. This is because, as a molecule gets longer, the total binding forces between molecules are greater, making the polymer chain stronger. By Sid Perkins. October 13, at am.
Polymers are everywhere. Just look around. Your plastic water bottle. The nylon and polyester in your jacket or sneakers. The rubber in the tires on the family car. Now take a look in the mirror. Many proteins in your body are polymers, too.
Consider keratin KAIR-uh-tin , the stuff your hair and nails are made from. Even the DNA in your cells is a polymer. By definition, polymers are large molecules made by bonding chemically linking a series of building blocks. Think of a polymer as a chain, with each of its links a monomer. Those monomers can be simple — just an atom or two or three — or they might be complicated ring-shaped structures containing a dozen or more atoms.
But in proteins, DNA and other natural polymers, links in the chain often differ from their neighbors. In some cases, polymers form branching networks rather than single chains. Polymer Formula Polyamide 6 Polyamide 6,6 The polyester terylene formed from the dimethyl ester of 1,4-benzenedicarboxylic acid and ethane-1,2-diol Poly propenonitrile Poly ethene Poly propene Poly chloroethene Poly tetrafluoroethene Table 4 Some polymers used to make fibres.
A simple example of a polymer with a side chain is poly propene. The propene molecule is asymmetrical, and, when polymerized, can form three basic chain structures dependent on the position of the methyl groups: two are stereoregular isotactic and syndiotactic and the third does not have a regular structure and is termed atactic as shown diagrammatically below: Figure 5 Molecular structures of poly propene. The syndiotactic polymer, because of its regular structure, is also crystalline.
Manufacture of polymers As discussed above, polymers can be characterised by the method of polymerization, addition and condensation. Catalysts for polymerization reactions Ziegler-Natta catalysts Ziegler-Natta catalysts are organometallic compounds, prepared from titanium compounds with an aluminium trialkyl which acts as a promoter: The alkyl groups used include ethyl, hexyl and octyl.
The role of the titanium catalyst can be represented as shown in Figure 6. Radical polymerization Many polymers, including all of the addition polymers in Table 1, are produced using radical initiators, which act as catalysts. For example the polymerization of chloroethene is started by warming it with a minute trace of a peroxide R-O-O-R : Figure 7 A mechanism for the free radical polymerization of chloroethene to poly chloroethene.
The polymer radical can also abstract a hydrogen atom from its own chain: Both of these reactions lead to side chains so that the molecules of the polymer cannot pack together in a regular way.
Plastics formulation The properties of many plastics can be modified by varying their formulation. Figure 8 Specific properties can be produced by mixing polymers.
For example this shirt is made from a mixture of poly propenoate acrylic , aramid and polyamide fibres which gives protection against heat and yet remains comfortable to wear. By kind permission of DuPont. Additive Examples Function Plasticiser e. Large amounts give a flexible product, low quantities produce a rigid one.
Stabiliser e. Without a stabiliser, poly chloroethene , for example, decomposes on heating to give a brittle product and hydrogen chloride. Some plastics become coloured yellowing when exposed to long periods of sunlight.
Extender Chlorinated hydrocarbons Extends the effect of the plasticiser, but generally cannot plasticise alone. They are cheaper than plasticisers, so help reduce costs.
Fillers Chalk, glass fibre Tailor the plastic for special requirements, or make it cheaper. Miscellaneous Flame retardants, UV stabilisers, antistatics, processing aids, pigments Impart specially required properties to the plastic for the manufacturing process or for end-use. Processing plastics Processing converts plastics into useful articles. Processing methods are given in Table 6. Process Application Compression moulding Usually for thermosets - powder moulded under heat and pressure. Injection moulding Usually for thermoplastics - molten plastics injected into a mould under pressure.
The mould surface detail can be accurately reproduced. Very widely used. Rotational moulding Usually for thermoplastics. The powder is heated in a closed mould which is rotated, fairly slowly, simultaneously about two axes. Surface detail is poor but this method can be used to make large hollow articles.
Reaction injection moulding Usually for thermosets, polymerization takes place in the mould thereby producing the finished article directly from a resin. Extrusion Usually for thermoplastics - the molten plastics are fed by a screw through a die, which for sheet or film, for instance, is a slit.
Various extensions to the process are possible - e. Calendering Usually for thermoplastics - molten plastics squeezed between hot rollers to form foil and sheet. Thermoforming Heat-softened thermoplastic sheet is drawn into or over a mould. If a vacuum is used to 'suck' the sheet into a mould, the process is known as vacuum forming. This process is used for a variety of articles, ranging from chocolate box liners to acrylic baths. Polymers: their manufacture and uses While this unit is concerned with the general principles underlying the structure, formulation and processing of polymers produced today, the manufacture and properties of the polymers vary considerably.
The following are discussed in other units: Methanal plastics Polyamides Polycarbonates Poly chloroethene Polyesters Poly ethene Poly methyl 2-methylpropenoate Poly phenylethene Poly propene Poly propenoic acid Poly propenonitrile Poly tetrafluoroethene Polyurethanes Silicones Important developments in recent years include degradable plastics and methods of recycling polymers which include reusing the polymer and degradation followed by repolymerization.
Date last amended: 18th March With all these advantages it is not surprising that much of what you see around you is plastic.
Indeed, the low cost, light weight, strength and design adaptability of plastics to meet a variety of applications have resulted in strong year after year growth in their production and use, which is likely to continue.
Indeed, many plastics are employed in disposable products meant only for a single use. Successful solutions to technological projects are often achieved by focusing on a limited set of variables that are directly linked to a desired outcome. However, nature often has a way of rewarding such success by exposing unexpected problems generated "outside the box" of the defined project. In the case of plastics, their advantageous durability and relative low cost have resulted in serious environmental pollution as used items and wrappings are casually discarded and replaced in a never ending cycle.
We see this every day on the streets and fields of our neighborhoods, but the problem is far more dire. Charles Moore, an American oceanographer, in discovered an enormous stew of trash, estimated at nearly million tons, floating in the Pacific Ocean between San Francisco and Hawaii.
The information provided here, and the illustration on the left, come from an article by Susan Casey in BestLife Clock-wise circulation of currents driven by the global wind system and constrained by surrounding continents form a vortex or gyre comparable to a large whirlpool.
The larger North Pacific Subtropical Gyre, referred to as the doldrums, is the convergence zone where plastic and other waste mixes together. Aside from its disgusting aesthetic presence, the garbage patch is representative of serious environmental and health problems.
No one knows how long it will take for some of these plastics to biodegrade, or return to their component molecules. Persistent objects such as six-pack rings and discarded nets trap sea animals. Smaller plastic scraps are mistaken for food by sea birds; and are often found undigested in the gut of dead birds. Nurdles, lentil-size pellets of plastic, found in abundance where plastics are manufactured and distributed, are dispersed by wind throughout the biosphere.
They're light enough to blow around like dust and to wash into harbors, storm drains, and creeks. Escaped nurdles and other plastic litter migrate to the ocean gyre largely from land.
At places as remote as Rarotonga, in the Cook Islands they're commonly found mixed with beach sand. Once in the ocean, nurdles may absorb up to a million times the level of any organic pollutants found in surrounding waters. Nurdles in the sea are easily mistaken for fish eggs by creatures that would very much like to have such a snack. Once inside the body of a bigeye tuna or a king salmon, they become part of our food chain. Most plastics crumble into ever-tinier fragments as they are exposed to sunlight and the elements.
Except for the small amount that's been incinerated—and it's a very small amount—every bit of plastic ever made still exists, unless the material's molecular structure is designed to favor biodegradation.
Unfortunately, cleaning up the garbage patch is not a realistic option, and unless we change our disposal and recycling habits, it will undoubtedly get bigger. One sensible solution would require manufacturers to use natural biodegradable packaging materials whenever possible, and consumers to conscientiously dispose of their plastic waste. Thus, instead of consigning all plastic trash to a land fill, some of it may provide energy by direct combustion, and some converted for reuse as a substitute for virgin plastics.
The latter is particularly attractive since a majority of plastics are made from petroleum, a diminishing resource with a volatile price. The energy potential of plastic waste is relatively significant, ranging from The use of plastic waste as a fuel source would be an effective means of reducing landfill requirements while recovering energy.
This, however, depends on using appropriate materials. Inadequate control of combustion, especially for plastics containing chlorine, fluorine and bromine, constitutes a risk of emitting toxic pollutants. Whether used as fuels or a source of recycled plastic, plastic waste must be separated into different categories. To this end, an identification coding system was developed by the Society of the Plastics Industry SPI in , and is used internationally. This code, shown on the right, is a set of symbols placed on plastics to identify the polymer type, for the purpose of allowing efficient separation of different polymer types for recycling.
The abbreviations of the code are explained in the following table. Despite use of the recycling symbol in the coding of plastics, there is consumer confusion about which plastics are readily recyclable. However, some regions are expanding the range of plastics collected as markets become available.
Los Angeles, for example, recycles all clean plastics numbered 1 through 7 In theory, most plastics are recyclable and some types can be used in combination with others. In many instances, however, there is an incompatibility between different types that necessitates their effective separation. Since the plastics utilized in a given manufacturing sector e. The plastic trash from most households, even with some user separation, is a mixture of unidentified pieces.
Recycling of such mixtures is a challenging problem. When placed in a medium of intermediate density, particles of different densities separate-lower density particles float while those of higher density sink.
Various separation media have been used, including water or water solutions of known density alcohol, NaCl, CaCl 2 or ZnCl 2. As shown in the following table, the densities of common plastics differ sufficiently to permit them to be discriminated in this fashion. The cylindroconical cyclone device, shown on the right, provides a continuous feed procedure in which the material to be separated is pumped into the vessel at the same time as the separating media.
Some polymers, such as polystyrene and polyurethane, are commonly formed into foamed solids that have a much lower density than the solid material. One serious problem in recycling is posed by the many additives found in plastic waste.
These include pigments for coloring, solid fibers in composites, stabilizers and plasticizers. In the case of PETE or PET , which is commonly used for bottles, some waste may be mechanically and thermally treated to produce low grade packaging materials and fibers.
To increase the value of recovered PETE it may be depolymerized by superheated methanol into dimethyl terephthalate and ethylene glycol. These chemicals are then purified and used to make virgin PETE.
Hydrocarbon polymers such as polyethylene and polypropylene may be melted and extruded into pellets for reuse. However, the presence of dyes or pigments limits the value of this product.
Plastics derived from natural materials, such as cellulose, starch and hydroxycarboxylic acids are more easily decomposed when exposed to oxygen, water, soil organisms and sunlight than are most petroleum based polymers.
The glycoside linkages in polysaccharides and the ester groups in polyesters represent points of attack by the enzymes of microorganisms that facilitate their decomposition. Such biodegradable materials can be composted, broken down and returned to the earth as useful nutrients. However, it is important to recognize that proper composting is necessary. Placing such materials in a landfill results in a slower anaerobic decomposition, which produces methane, a greenhouse gas.
Derivatives of cellulose, such as cellulose acetate , have long served for the manufacture of films and fibers. The most useful acetate material is the diacetate, in which two thirds of the cellulose hydroxyl groups have been esterified. Acetate fibers loose strength when wet, and acetate clothing must be dry cleaned. The other major polysaccharide, starch, is less robust than cellulose, but in pelletized form it is now replacing polystyrene as a packing material.
The two natural polyesters that are finding increasing use as replacements for petroleum based plastics are polylactide PLA and polyhydroxyalkanoates PHA , the latter most commonly as copolymers with polyhydroxybutyrate PHB. Structures for the these polymers and their monomer precursors are shown below.
PLA is actually a polymer of lactic acid, but the dimeric lactide is used as the precursor to avoid the water that would be formed in a direct poly-esterification. Bacterial fermentation is used to produce lactic acid from corn starch or cane sugar. After dimerization to the lactide, ring-opening polymerization of the purified lactide is effected using stannous compounds as catalysts. PLA can be processed like most thermoplastics into fibers and films. In situations that require a high level of impact strength, the toughness of PLA in its pristine state is often insufficient.
Blends of PLA with polymers such as ABS have good form-stability and visual transparency, making them useful for low-end packaging applications. PLA materials are currently used in a number of biomedical applications, such as sutures, stents, dialysis media and drug delivery devices. However, one of the drawbacks of polylactides for biomedical applications is their brittleness. Due to the chiral nature of lactic acid, several distinct forms of polylactide exist.
PHA polyhydroxyalkanoates are synthesized by microorganisms such as Alcaligenes eutrophus , grown in a suitable medium and fed appropriate nutrients so that it multiplies rapidly. Once the population has increased, the nutrient composition is changed, forcing the micro-organism to synthesize PHA.
Pure PHB, consisting of to hydroxy acid units, is relatively brittle and stiff. Depending upon the microorganism, many of which are genetically engineered for this purpose, and the cultivation conditions, homo- or copolyesters with different hydroxyalkanic acids may be generated. Such copolymers may have improved physical properties compared with homo P 3HB.
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