Group items tagged

a source of energy

building blocks for polysaccharides (giant carbohydrates

components of other molecules eg DNA, RNA, glycolipids, glycoproteins, ATP

Monosaccharides are the simplest carbohydrates and are often called single sugars.

Monosaccharides have the general molecular formula (CH2O)n, where n can be 3, 5 or 6.

n = 3 trioses, e.g. glyceraldehyde n = 5 pentoses, e.g. ribose and deoxyribose ('pent' indicates 5) n = 6 hexoses, e.g. fructose, glucose and galactose ('hex' indicates 6)

Molecules that have the same molecular formula but different structural formulae are called structural isomers.

Monosaccharides containing the aldehyde group are classified as aldoses, and those with a ketone group are classified as ketoses. Aldoses are reducing sugars; ketoses are non-reducing sugars.

in water pentoses and hexoses exist mainly in the cyclic form, and it is in this form that they combine to form larger saccharide molecules.

There are two forms of the cyclic glucose molecule: α-glucose and β-glucose.

Two glucose molecules react to form the dissacharide maltose. Starch and cellulose are polysaccharides made up of glucose units.

Galactose molecules look very similar to glucose molecules. They can also exist in α and β forms. Galactose reacts with glucose to make the dissacharide lactose.

However, glucose and galactose cannot be easily converted into one another. Galactose cannot play the same part in respiration as glucose.

Fructose reacts with glucose to make the dissacharide sucrose.

Ribose and deoxyribose are pentoses. The ribose unit forms part of a nucleotide of RNA. The deoxyribose unit forms part of the nucleotide of DNA.

Monosaccharides are rare in nature. Most sugars found in nature are disaccharides. These form when two monosaccharides react.

The three most important disaccharides are sucrose, lactose and maltose.

Disaccharides are soluble in water, but they are too big to pass through the cell membrane by diffusion.

This is a hydrolysis reaction and is the reverse of a condensation reaction. It releases energy.

Monosaccharides are converted into disaccharides in the cell by condensation reactions. Further condensation reactions result in the formation of polysaccharides. These are giant molecules which, importantly, are too big to escape from the cell. These are broken down by hydrolysis into monosaccharides when energy is needed by the cell.

Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers. The properties of a polysaccharide molecule depend on: its length (though they are usually very long) the extent of any branching (addition of units to the side of the chain rather than one of its ends) any folding which results in a more compact molecule whether the chain is 'straight' or 'coiled'

Starch is often produced in plants as a way of storing energy. It exists in two forms: amylose and amylopectin

Amylose is an unbranched polymer of α-glucose. The molecules coil into a helical structure. It forms a colloidal suspension in hot water. Amylopectin is a branched polymer of α-glucose. It is completely insoluble in water.

Glycogen is amylopectin with very short distances between the branching side-chains.

Inside the cell, glucose can be polymerised to make glycogen which acts as a carbohydrate energy store.

Cellulose is a third polymer made from glucose. But this time it's made from β-glucose molecules and the polymer molecules are 'straight'.

Cellulose serves a very different purpose in nature to starch and glycogen. It makes up the cell walls in plant cells. These are much tougher than cell membranes. This toughness is due to the arrangement of glucose units in the polymer chain and the hydrogen-bonding between neighbouring chains.

Cellulose is not hydrolysed easily and, therefore, cannot be digested so it is not a source of energy for humans.

Biopolymer Natural Source What is it? Lactic Acid Beets, corn, potatoes, and others Produced through fermentation of sugar feedstocks, such as beets, and by converting starch in corn, potatoes, or other starch sources. It is polymerized to produce polylactic acid -- a polymer that is used to produce plastic. Triglycerides Vegetable oils These form a large part of the storage lipids found in plant and animal cells. Vegetable oils are one possible source of triglycerides that can be polymerized into plastics.

Using Fermentation to Produce Plastics Fermentation, used for hundreds of years by humans, is even more powerful when coupled with new biotechnology techniques.

Today, fermentation can be carried out with genetically engineered microorganisms, specially designed for the conditions under which fermentation takes place,

Fermentation, in fact, is the process by which bacteria can be used to create polyesters. Bacteria called Ralstonia eutropha are used to do this. The bacteria use the sugar of harvested plants, such as corn, to fuel their cellular processes. The by-product of these cellular processes is the polymer.

Lactic acid is fermented from sugar, much like the process used to directly manufacture polymers by bacteria. However, in this fermentation process, the final product of fermentation is lactic acid, rather than a polymer. After the lactic acid is produced, it is converted to polylactic acid using traditional polymerization processes.

Plants are becoming factories for the production of plastics. Researchers created a Arabidopis thaliana plant through genetic engineering. The plant contains the enzymes used by bacteria to create plastics. Bacteria create the plastic through the conversion of sunlight into energy. The researchers have transferred the gene that codes for this enzyme into the plant, as a result the plant produces plastic through its cellular processes. The plant is harvested and the plastic is extracted from it using a solvent. The liquid resulting from this process is distilled to separate the solvent from the plastic.

Currently, fossil fuel is still used as an energy source during the production process. This has raised questions by some regarding how much fossil fuel is actually saved by manufacturing bioplastics. Only a few processes have emerged that actually use less energy in the production process.

Energy use is not the only concern when it comes to biopolymers and bioplastics. There are also concerns about how to balance the need to grow plants for food, and the need to grow plants for use as raw materials. Agricultural space needs to be shared. Researchers are looking into creating a plant that can be used for food, but also as feedstock for plastic production.

Biopolymers and bioplastics are the main components in creating a sustainable plastics industry. These products reduce the dependence on non-renewable fossil fuels, and are easily biodegradable. Together, this greatly limits the environmental impacts of plastic use and manufacture. Also, characteristics such as being biodegradable make plastics more acceptable for long term use by society. It is likely that in the long term, these products will mean plastics will remain affordable, even as fossil fuel reserves diminish.

Most synthetic polymers are made from the non-renewable resource, petroleum, and as such, the "age of plastics" is limited unless other ways are found to make them. Since most polymers have carbon atoms as the basis of their structure, in theory at least, there are numerous materials that could be used as starting points.

Disposing of plastics is also a serious problem, both because they contribute to the growing mounds of garbage accumulating everyday and because most are not biodegradable. Researchers are busy trying to find ways to speed-up the decomposition time which, if left to occur naturally, can take decades.

n order for monomers to chemically combine with each other and form long chains, there must be a mechanism by which the individual units can join or bond to each other. One method by which this happens is called addition because no atoms are gained or lost in the process. The monomers simply "add" together and the polymer is called an addition polymer.

The simplest chemical structure by which this can happen involves monomers that contain double bonds (sharing two pairs of electrons). When the double bond breaks and changes into a single bond, each of the other two electrons are free and available to join with another monomer that has a free electron. This process can continue on and on. Polyethylene is an example of an addition polymer.

The polymerization process can be started by using heat and pressure or ultraviolet light or by using another more reactive chemical such as a peroxide. Under these conditions the double bond breaks leaving extremely reactive unpaired electrons called free radicals. These free radicals react readily with other free radicals or with double bonds and the polymer chain starts to form.

ifferent catalysts yield polymers with different properties because the size of the molecule may vary and the chains may be linear, branched, or cross-linked. Long linear chains of 10,000 or more monomers can pack very close together and form a hard, rigid, tough plastic known as high-density polyethylene or HDPE

Shorter, branched chains of about 500 monomers of ethylene cannot pack as closely together and this kind of polymer is known as low-density polyethylene or LDPE.

The ethylene monomer has two hydrogen atoms bonded to each carbon for a total of four hydrogen atoms that are not involved in the formation of the polymer. Many other polymers can be formed when one or more of these hydrogen atoms are replaced by some other atom or group of atoms.

Natural and synthetic rubbers are both addition polymers. Natural rubber is obtained from the sap that oozes from rubber trees. It was named by Joseph Priestley who used it to rub out pencil marks, hence, its name, a rubber. Natural rubber can be decomposed to yield monomers of isoprene.

It was sticky and smelly when it got too hot and it got hard and brittle in cold weather. These undesirable properties were eliminated when, in 1839, Charles Goodyear accidentally spilled a mixture of rubber and sulfur onto a hot stove and found that it did not melt but rather formed a much stronger but still elastic product. The process, called vulcanization, led to a more stable rubber product that withstood heat (without getting sticky) and cold (without getting hard) as well as being able to recover its original shape after being stretched. The sulfur makes cross-links in the long polymer chain and helps give it strength and resiliency, that is, if stretched, it will spring back to its original shape when the stress is released.

A second method by which monomers bond together to form polymers is called condensation.

Unlike addition polymers, in which all the atoms of the monomers are present in the polymer, two products result from the formation of condensation polymers, the polymer itself and another small molecule which is often, but not always, water.

One of the simplest of the condensation polymers is a type of nylon called nylon 6.

All amino acids molecules have an amine group (NH2) at one end and a carboxylic acid (COOH) group at the other end. A polymer forms when a hydrogen atom from the amine end of one molecule and an oxygen-hydrogen group (OH) from the carboxylic acid end of a second molecule split off and form a water molecule. The monomers join together as a new chemical bond forms between the nitrogen and carbon atoms. This new bond is called an amide linkage.

The new molecule, just like each of the monomers from which it formed, also has an amine group at one end (that can add to the carboxylic acid group of another monomer) and it has a carboxylic acid group at the other end (that can add to the amine end of another monomer). The chain can continue to grow and form very large polymers.

Polymers formed by this kind of condensation reaction are referred to as polyamides.

Nylon became a commercial product for Du Pont when their research scientists were able to draw it into long, thin, symmetrical filaments. As these polymer chains line up side-by-side, weak chemical bonds called hydrogen bonds form between adjacent chains. This makes the filaments very strong.

Another similar polymer of the polyamide type is the extremely light-weight but strong material known as Kevlar. It is used in bullet-proof vests, aircraft, and in recreational uses such as canoes. Like nylon, one of the monomers from which it is made is terephthalic acid. The other one is phenylenediamine.

Polyesters are another type of condensation polymer, so-called because the linkages formed when the monomers join together are called esters.

Probably the best known polyester is known by its trade name, Dacron.

Dacron is used primarily in fabrics and clear beverage bottles. Films of Dacron can be coated with metallic oxides, rolled into very thin sheets (only about one-thirtieth the thickness of a human hair), magnetized, and used to make audio and video tapes. When used in this way, it is extremely strong and goes by the trade name Mylar. Because it is not chemically reactive, and is not toxic, allergenic, or flammable, and because it does not promote blood-clotting, it can be used to replace human blood vessels when they are severely blocked and damaged or to replace the skin of burn victims.

Polysaccharides make ideal storage molecules for energy for a number of reasons; a) they are large, this makes them insoluble in water and therefore they exert no osmotic or chemical effect on the cell; b) they fold into compact shapes; c) they are easily converted into the required sugars when needed.

Glycogen is a branched polysaccharide found in nearly all animal cells and in certain protozoa and algae.

In humans and other vertebrates it is principally stored in the liver and muscles and is the main form of stored carbohydrate in the body, acting as a reservoir of glucose

Starch is similar to glycogen, however it is found in plant cells, protists and certain bacteria.

The starch granules are made up of two polysaccharides, amylose and amylopectin. Amylose is an unbranched molecule made up of several thousand glucose units, coiled helically into a more compact shape. Amylopectin is also compact but has a branched structure and is made up of twice as many glucose units as amylose.

For example, peptidoglycans, which are a combination of protein and polysaccharide and are found in the cell wall of certain bacteria. Glycolipids, a combination of polysaccharides and lipids are found in the cell membrane.

Other examples of thermoplastics are: Polyethylene: packaging, electrical insulation, milk and water bottles, packaging film, house wrap, agricultural film Polypropylene: carpet fibers, automotive bumpers, microwave containers, external prostheses Polyvinyl chloride (PVC): sheathing for electrical cables, floor and wall coverings, siding, credit cards, automobile instrument panels

These monomers are then chemically bonded into chains called polymers.

The resulting resins may be molded or formed to produce several different kinds of plastic products with application in many major markets. The variability of resin permits a compound to be tailored to a specific design or performance requirement.

Polymers are created by the chemical bonding of many identical or related basic units and those produced from a single monomer type are called homopolymers. These polymers are specifically made of small units bonded into long chains. Carbon makes up the backbone of the molecule and hydrogen atoms are bonded along the carbon backbone.

In order to achieve a commercial product, the plastic is subject to further treatment and the inclusion of additives which are selected to give it specified properties

Additives are incorporated into polymers to alter and improve their basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer properties as flow; to provide product color; and to provide special characteristics such as improved surface appearance or reduced friction.

Types of Additives:     antioxidants: for outside application,      colorants: for colored plastic parts, foaming agents: for Styrofoam cups,      plasticizers: used in toys and food processing equipment

A Thermoset is a polymer that solidifies or "sets" irreversibly when heated. Similar to the relationship between a raw and a cooked egg, once heated, a thermoset polymer can't be softened again and once cooked, the egg cannot revert back to its original form.

A Thermoplastic is a polymer in which the molecules are held together by weak secondary bonding forces that soften when exposed to heat and return to its original condition when cooled back down to room temperature. When a thermoplastic is softened by heat, it can then be shaped by extrusion, molding or pressing. Ice cubes are a common household item which exemplify the thermoplastic principle. Ice will melt when heated but readily solidifies when cooled.

In this method, a separate molding and cooling station on the equipment allows the parison to be continuously formed.  This technique is used mainly for small thin-walled parts ranging up to containers with five gallon capacities.  Parison programming can be used to vary the wall thickness.  Continuous extrusion also allows the use of heat-sensitive materials due to streamlined flow areas and die designs.

This technique is performed in three basic ways --reciprocating, ram accumulator, and accumulator head systems.  All three vary in machine design and the flow of molten resin through the die for parison forming.  However, each system is designed to produce larger, heavier, and thicker parts than continuous extrusion.

Blow moldable grades of material are initially injection molded into preform shapes.  These preforms are then thermally conditioned and then stretched (utilizing pneumatically operated stretch rods) low pressure air, followed by high pressure air up to 40 bar to form axially oriented parts with molded in necks.  The process is used to manufacture PET bottles.

This process utilizes various thermoplastic materials in a solid pelletized state and converts these materials by way of heat, pressure and compressed air into a finished good stat.The pellitized raw material is conveyed to the feed section of a plasticating extruder by way of a vacuum loader or auger screw.  The raw material is then conveyed forward through the extruder and is plastisized to a molten state of between 350 degrees and 500 degrees F. by way of a feed screw and external heating elements.The material in a melt state is then reshaped into a round hollow geometry termed a parison.  This parison is then extruded vertically from the head section of the machine through a round die at various outside and inside diameters.After extrusion of the parison between the two halves of a mold the press section closes encapsulating the parison inside the mold halves.  Upon mold close compressed air is entered into the parison by way of a centrally located air pipe or by piercing air needles.The molds are chilled with cooled water which transfers the hear form the now formed part inside the mold.  Upon complete part cooling the press section opens and the finished product is removed.  The material which is pinched off outside the mold cavity, or the flash, is then fed into a granulator which cops the flash into a granule size which can be fed back to the feed section of the extruder.

Table 7 shows that the composition varies from chains of only monomer A (homopolymer A) to chains containing only monomer B (homopolymer B).

In many of the structures no regularity can be detected, although there will be short sequences of one type of unit, and the copolymer can be regarded as completely random; such copolymers are usually said to be ideal copolymers. These possible copolymer structures are shown schematically in Table 7.

It can be shown that the rate of change of monomer concentration in any copolymerization is given by the equation where [M1] and [M2] are the concentrations of monomers 1 and 2 at any instant and r1 and r2., are reactivity ratios. The reactivity ratios represent the rate at which one type of growing chain end adds on to a monomer of the same structure relative to the rate at which it adds on to the alternative monomer. The copolymer equation can be used to predict chain structure in the three different ways, already mentioned.

An ideal copolymer will tend to form when each type of chain end shows an equal preference for adding on to either monomer. In this case, and the copolymer equation becomes Hence composition depends on the relative amounts of monomer present at any time and the relative reactivities of the two monomers.

Step growth copolymerizations produce ideal (random) copolymers since in this special case r 1 = r 2 = 1.

The main reason for copolymerizing different monomers is to adjust the physical properties of a given homopolymer to meet a specific demand. SBR elastomer, for example (Table 1), based on 24 wt% styrene monomer shows better mechanical properties and better resistance to degradation than polybutadiene alone

A second reason for copolymerization is to enhance the chemical reactivity of a polymer, particularly to aid crosslinking. Conventional vulcanization in rubbers is brought about by forming sulphur crosslinks at or near double bonds in the chain

To show the dramatic effect of copolymer structure on physical properties, consider the change from random SBR copolymer to a block copolymer of exactly the same chemical composition but where the styrene and butadiene parts are effectively homopolymer chains linked at two points: The material behaves like a vulcanized butadiene rubber without the need for chemical crosslinking since the styrene chains segregate together to form small islands or domains within the structure. Such so-called thermoplastic elastomers (TPEs) today form an important growth area for new polymers because of the process savings in manufacture that can be achieved with their use.

Polyethylene terephthalate (PET), or polyethylene terephthalic ester (PETE), is a condensation polymer produced from the monomers ethylene glycol, HOCH2CH2OH, a dialcohol, and dimethyl terephthalate, CH3O2C–C6H4–CO2CH3, a diester. By the process of transesterification, these monomers form ester linkages between them, yielding a polyester

PETE fibers are manufactured under the trade names of Dacron and Fortrel.

Pleats and creases can be permanently heat set in fabrics containing polyester fibers, so-called permanent press fabrics. PETE can also be formed into transparent sheets and castings.

Transparent 2-liter carbonated beverage bottles are made from PETE.

ne form of PETE is the hardest known polymer and is used in eyeglass lenses.

     Polyethylene is perhaps the simplest polymer, composed of chains of repeating –CH2– units. It is produced by the addition polymerization of ethylene, CH2=CH2 (ethene)

HDPE is hard, tough, and resilient. Most HDPE is used in the manufacture of containers, such as milk bottles and laundry detergent jugs.

LDPE is relatively soft, and most of it is used in the production of plastic films, such as those used in sandwich bags.

Polymerization of vinyl chloride, CH2=CHCl (chloroethene), produces a polymer similar to polyethylene, but having chlorine atoms at alternate carbon atoms on the chain.

About two-thirds of the PVC produced annually is used in the manufacture of pipe. It is also used in the production of “vinyl” siding for houses and clear plastic bottles.

is used to form flexible articles such as raincoats and shower curtains.

This polymer is produced by the addition polymerization of propylene, CH2=CHCH3 (propene). Its molecular structure is similar to that of polyethylene, but has a methyl group (–CH3) on alternate carbon atoms of the chain.

olypropylene is used extensively in the automotive industry for interior trim, such as instrument panels, and in food packaging, such as yogurt containers. It is formed into fibers of very low absorbance and high stain resistance, used in clothing and home furnishings, especially carpeting.

Styrene, CH2=CH–C6H5, polymerizes readily to form polystyrene (PS), a hard, highly transparent polymer.

A large portion of production goes into packaging. The thin, rigid, transparent containers in which fresh foods, such as salads, are packaged are made from polystyrene. Polystyrene is readily foamed or formed into beads. These foams and beads are excellent thermal insulators and are used to produce home insulation and containers for hot foods. Styrofoam is a trade name for foamed polystyrene.

eflon is a trade name of polytetrafluoroethylene, PTFE. It is formed by the addition polymerization of tetrafluoroethylene, CF2=CF2 (tetrafluoroethene). PTFE is distinguished by its complete resistance to attack by virtually all chemicals and by its slippery surface. It maintains its physical properties over a large temperature range, -270° to 385°C. These properties make it especially useful for components that must operate under harsh chemical conditions and at temperature extremes. Its most familiar household use is as a coating on cooking utensils.

his important class of polymers is formed by the addition polymerization of an diisocyanate (whose molecules contain two –NCO groups) and a dialcohol (two –OH groups).

Polyurethane is spun into elastic fibers, called spandex, and sold under the trade name Lycra. Polyurethane can also be foamed. Soft polyurethane foams are used in upholstery, and hard foams are used structurally in light aircraft wings and sail boards.

Polyamides are a group of condensation polymers commonly known as nylon. Nylon is made from two monomers, one a dichloride and the other a diamine.

Nylon can be readily formed into fibers that are strong and long wearing, making them well suited for use in carpeting, upholstery fabric, tire cords, brushes, and turf for athletic fields. Nylon is also formed into rods, bars, and sheets that are easily formed and machined.

Polyacrylamide is a condensation polymer with an unusual and useful property.

This produces a network of polymer chains, rather like a tiny sponge. The free, unlinked amide groups, because they contain –NH2 groups, can form hydrogen bonds with water. This gives the tiny cross linked sponges a great affinity for water. Polyacrylamide can absorb many times its mass in water. T

his property is useful in a variety of applications, such as in diapers and in potting soil. The polyacrylamide will release the absorbed water if a substance that interferes with hydrogen bonding is added. Ionic substances, such as salt, cause polyacrylamide to release its absorbed water.

Over the past few decades, the use of polymers in disposable consumer goods has grown tremendously. This growth is proving to be taxing on the waste disposal system, consuming a large fraction of available landfill space.

To help sort wastes by type of polymer, most disposable polymeric goods are labeled with a recycling code: three arrows around a number above the polymer's acronym. These are intended to help consumers separate the waste polymers according to type before disposing of them. In the city of Madison, currently only type 1 (PETE) and type 2 (HDPE) polymers are being recycled – see below. The recycling of polymers is not a closed loop, where a material is reformed into new products repeatedly, such as in the case with aluminum. Most polymeric materials are recycled only once, and the product made of recycled polymer is discarded after use

General Rules Remove and discard all lids or caps. Rinse all containers. Remove and discard sprayer tops. CRUSH all plastic bottles to save space. No 5 gallon pails. No containers with metal handles.

What can be Recycled?Plastic Code Number Recyclable Containers Soda Bottles Water Bottles Juice Bottles Cooking Oil Bottles Soap/Detergent Bottles Shampoo Bottles Clear Liquor Bottles Food Jars (Peanut Butter etc.) Plastic Code Number Recyclable Containers Milk Bottles Water Bottles Juice Bottles Cooking Oil Containers Windshield Washer Fluid Bottles Shampoo Bottles Butter/Margarine Tubs Cottage Cheese Containers Ice Cream Containers Without Metal Handles Baby Wipe Containers Do NOT Recycle This Plastic 1. Automotive Product Containers Including: Motor Oil Bottles Anti-Freeze Containers Gasoline and Oil Additive Bottles 2. Brown Liquor Bottles 3. All Containers Marked With The Following Codes:            

For sugars with more than one chiral center, the D or L designation refers to the asymmetric carbon farthest from the aldehyde or keto group. Most naturally occurring sugars are D isomers.

The 9 atoms that make up the fused rings (5 carbon, 4 nitrogen) are numbered 1-9. All ring atoms lie in the same plane.

The deoxyribose sugar of the DNA backbone has 5 carbons and 3 oxygens. The carbon atoms are numbered 1', 2', 3', 4', and 5' to distinguish from the numbering of the atoms of the purine and pyrmidine rings. The hydroxyl groups on the 5'- and 3'- carbons link to the phosphate groups to form the DNA backbone

A nucleoside is one of the four DNA bases covalently attached to the C1' position of a sugar.

Nucleosides differ from nucleotides in that they lack phosphate groups. The four different nucleosides of DNA are deoxyadenosine (dA), deoxyguanosine (dG), deoxycytosine (dC), and (deoxy)thymidine (dT, or T).

A nucleotide is a nucleoside with one or more phosphate groups covalently attached to the 3'- and/or 5'-hydroxyl group(s).

The DNA backbone is a polymer with an alternating sugar-phosphate sequence. The deoxyribose sugars are joined at both the 3'-hydroxyl and 5'-hydroxyl groups to phosphate groups in ester links, also known as "phosphodiester" bonds.

DNA is a normally double stranded macromolecule. Two polynucleotide chains, held together by weak thermodynamic forces, form a DNA molecule.

Two DNA strands form a helical spiral, winding around a helix axis in a right-handed spiral The two polynucleotide chains run in opposite directions The sugar-phosphate backbones of the two DNA strands wind around the helix axis like the railing of a sprial staircase The bases of the individual nucleotides are on the inside of the helix, stacked on top of each other like the steps of a spiral staircase.

Within the DNA double helix, A forms 2 hydrogen bonds with T on the opposite strand, and G forms 3 hyrdorgen bonds with C on the opposite strand.

The first step in the chain-reaction polymerization process, initiation, occurs when the free-radical catalyst reacts with a double bonded carbon monomer, beginning the polymer chain. The double carbon bond breaks apart, the monomer bonds to the free radical, and the free electron is transferred to the outside carbon atom in this reaction.

The next step in the process, propagation, is a repetitive operation in which the physical chain of the polymer is formed. The double bond of successive monomers is opened up when the monomer is reacted to the reactive polymer chain. The free electron is successively passed down the line of the chain to the outside carbon atom.

Thermodynamically speaking, the sum of the energies of the polymer is less than the sum of the energies of the individual monomers. Simply put, the single bounds in the polymeric chain are more stable than the double bonds of the monomer.

Termination occurs when another free radical (R-O.), left over from the original splitting of the organic peroxide, meets the end of the growing chain. This free-radical terminates the chain by linking with the last CH2. component of the polymer chain. This reaction produces a complete polymer chain. Termination can also occur when two unfinished chains bond together. Both termination types are diagrammed below. Other types of termination are also possible.

This exothermic reaction occurs extremely fast, forming individual chains of polyethylene often in less than 0.1 second.

Step-reaction (condensation) polymerization is another common type of polymerization. This polymerization method typically produces polymers of lower molecular weight than chain reactions and requires higher temperatures to occur. Unlike addition polymerization, step-wise reactions involve two different types of di-functional monomers or end groups that react with one another, forming a chain. Condensation polymerization also produces a small molecular by-product (water, HCl, etc.).

As indicated above, both addition and condensation polymers can be linear, branched, or cross-linked. Linear polymers are made up of one long continuous chain, without any excess appendages or attachments. Branched polymers have a chain structure that consists of one main chain of molecules with smaller molecular chains branching from it. A branched chain-structure tends to lower the degree of crystallinity and density of a polymer. Cross-linking in polymers occurs when primary valence bonds are formed between separate polymer chain molecules. Chains with only one type of monomer are known as homopolymers. If two or more different type monomers are involved, the resulting copolymer can have several configurations or arrangements of the monomers along the chain. The four main configurations are depicted below:

They can be found in either crystalline or amorphous forms. Crystalline polymers are only possible if there is a regular chemical structure (e.g., homopolymers or alternating copolymers), and the chains possess a highly ordered arrangement of their segments. Crystallinity in polymers is favored in symmetrical polymer chains, however, it is never 100%. These semi-crystalline polymers possess a rather typical liquefaction pathway, retaining their solid state until they reach their melting point at Tm.

Amorphous polymers do not show order. The molecular segments in amorphous polymers or the amorphous domains of semi-crystalline polymers are randomly arranged and entangled. Amorphous polymers do not have a definable Tm due to their randomness

At low temperatures, below their glass transition temperature (Tg), the segments are immobile and the sample is often brittle. As temperatures increase close to Tg, the molecular segments can begin to move. Above Tg, the mobility is sufficient (if no crystals are present) that the polymer can flow as a highly viscous liquid.

Thermoplastics are generally carbon containing polymers synthesized by addition or condensation polymerization. This process forms strong covalent bonds within the chains and weaker secondary Van der Waals bonds between the chains. Usually, these secondary forces can be easily overcome by thermal energy, making thermoplastics moldable at high temperatures.

Thermosets have the same Van der Waals bonds that thermoplastics do. They also have a stronger linkage to other chains. Strong covalent bonds chemically hold different chains together in a thermoset material. The chains may be directly bonded to each other or be bonded through other molecules. This "cross-linking" between the chains allows the material to resist softening upon heating.

Compression Molding This type of molding was among the first to be used to form plastics. It involves four steps: Pre-formed blanks, powders or pellets are placed in the bottom section of a heated mold or die. The other half of the mold is lowered and is pressure applied. The material softens under heat and pressure, flowing to fill the mold. Excess is squeezed from the mold. If a thermoset, cross-linking occurs in the mold. The mold is opened and the part is removed. For thermoplastics, the mold is cooled before removal so the part will not lose its shape. Thermosets may be ejected while they are hot and after curing is complete. This process is slow

Injection Molding This very common process for forming plastics involves four steps: Powder or pelletized polymer is heated to the liquid state. Under pressure, the liquid polymer is forced into a mold through an opening, called a sprue. Gates control the flow of material. The pressurized material is held in the mold until it solidifies. The mold is opened and the part removed by ejector pins. Advantages of injection molding include rapid processing, little waste, and easy automation.

Transfer Molding This process is a modification of compression molding. It is used primarily to produce thermosetting plastics. Its steps are: A partially polymerized material is placed in a heated chamber. A plunger forces the flowing material into molds. The material flows through sprues, runners and gates. The temperature and pressure inside the mold are higher than in the heated chamber, which induces cross-linking. The plastic cures, is hardened, the mold opened, and the part removed. Mold costs are expensive and much scrap material collects in the sprues and runners, but complex parts of varying thickness can be accurately produced.

Extrusion This process makes parts of constant cross section like pipes and rods. Molten polymer goes through a die to produce a final shape. It involves four steps: Pellets of the polymer are mixed with coloring and additives. The material is heated to its proper plasticity. The material is forced through a die. The material is cooled.

Blow Molding Blow molding produces bottles, globe light fixtures, tubs, automobile gasoline tanks, and drums. It involves: A softened plastic tube is extruded The tube is clamped at one end and inflated to fill a mold. Solid shell plastics are removed from the mold. This process is rapid and relatively inexpensive.

In 1989, a billion pounds of virgin PET were used to make beverage bottles of which about 20% was recycled. Of the amount recycled, 50% was used for fiberfill and strapping. The reprocessors claim to make a high quality, 99% pure, granulated PET. It sells at 35 to 60% of virgin PET costs. The major reuses of PET include sheet, fiber, film, and extrusions.

Of the plastics that have a potential for recycling, the rigid HDPE container is the one most likely to be found in a landfill. Less than 5% of HDPE containers are treated or processed in a manner that makes recycling easy.

There is a great potential for the use of recycled HDPE in base cups, drainage pipes, flower pots, plastic lumber, trash cans, automotive mud flaps, kitchen drain boards, beverage bottle crates, and pallets.

LDPE is recycled by giant resin suppliers and merchant processors either by burning it as a fuel for energy or reusing it in trash bags. Recycling trash bags is a big business.

There is much controversy concerning the recycling and reuse of PVC due to health and safety issues. When PVC is burned, the effects on the incinerator and quality of the air are often questioned. The Federal Food and Drug Administration (FDA) has ordered its staff to prepare environmental impact statements covering PVC's role in landfills and incineration. The burning of PVC releases toxic dioxins, furans, and hydrogen chloride.

PVC is used in food and alcoholic beverage containers with FDA approval. The future of PVC rests in the hands of the plastics industry to resolve the issue of the toxic effects of the incineration of PVC. It is of interest to note that PVC accounts for less than 1% of land fill waste.

PS and its manufacturers have been the target of environmentalists for several years. The manufacturers and recyclers are working hard to make recycling of PS as common as that of paper and metals. One company, Rubbermaid, is testing reclaimed PS in service trays and other utility items.

Table 3: Major Plastic Resins and Their Uses Resin CodeResin NameCommon UsesExamples of Recycled Products Polyethylene Terephthalate (PET or PETE) Soft drink bottles, peanut butter jars, salad dressing bottles, mouth wash jars Liquid soap bottles, strapping, fiberfill for winter coats, surfboards, paint brushes, fuzz on tennis balls, soft drink bottles, film High density Polyethylene (HDPE) Milk, water, and juice containers, grocery bags, toys, liquid detergent bottles Soft drink based cups, flower pots, drain pipes, signs, stadium seats, trash cans, re-cycling bins, traffic barrier cones, golf bag liners, toys Polyvinyl Chloride or Vinyl (PVC-V) Clear food packaging, shampoo bottles Floor mats, pipes, hoses, mud flaps Low density Polyethylene (LDPE) Bread bags, frozen food bags, grocery bags Garbage can liners, grocery bags, multi purpose bags Polypropylene (PP) Ketchup bottles, yogurt containers, margarine, tubs, medicine bottles Manhole steps, paint buckets, videocassette storage cases, ice scrapers, fast food trays, lawn mower wheels, automobile battery parts. Polystyrene (PS) Video cassette cases, compact disk jackets, coffee cups, cutlery, cafeteria trays, grocery store meat trays, fast-food sandwich container License plate holders, golf course and septic tank drainage systems, desk top accessories, hanging files, food service trays, flower pots, trash cans