Chem 109H Fall08

Since only a fraction of certain types of plastic could realistically be captured by a curbside program, the net impact of initiating curbside collection could be an increase in the amount of plastic landfilled. The Berkeley pilot program showed no reduction of plastic being sent to the landfill in the areas where the curbside collection was in operation.

A chasing arrows symbol means a plastic container is recyclable. The arrows are meaningless. Every plastic container is marked with the chasing arrows symbol. The only information in the symbol is the number inside the arrows, which indicates the general class of resin used to make the container.

Packaging resins are made from petroleum refineries’ waste. Plastic resins are made from non-renewable natural resources that could be used for a variety of other applications or conserved. Most packaging plastics are made from the same natural gas used in homes to heat water and cook.

Using plastic containers conserves energy. When the equation includes the energy used to synthesize the plastic resin, making plastic containers uses as much energy as making glass containers from virgin materials, and much more than making glass containers from recycled materials. Using refillables is the most energy conservative.

Our choice is limited to recycling or wasting. Source reduction is preferable for many types of plastic and isn’t difficult. Opportunities include using refillable containers, buying in bulk, buying things that don’t need much packaging, and buying things in recyclable and recycled packages

Plastic packaging has economic, health, and environmental costs and benefits.

Plastic container producers do not use any recycled plastic in their packaging. Recycled content laws could reduce the use of virgin resin for packaging. Unfortunately, the virgin&endash;plastics industry has resisted such cooperation by strongly opposing recycled -content legislation, and has defeated or weakened consumer efforts to institute stronger laws.

Processing used plastics often costs more than virgin plastic. As plastic producers increase production and reduce prices on virgin plastics, the markets for used plastic are diminishing. PET recyclers cannot compete with the virgin resin flooding the market.

1. Reduce the useSource reduction Retailers and consumers can select products that use little or no packaging. Select packaging materials that are recycled into new packaging - such as glass and paper.

2. Reuse containersSince refillable plastic containers can be reused about 25 times, container reuse can lead to a substantial reduction in the demand for disposable plastic, and reduced use of materials and energy

3. Require producers to take back resins

Make reprocessing easier by limiting the number of container types and shapes, using only one type of resin in each container, making collapsible containers, eliminating pigments, using water-dispersible adhesives for labels, and phasing out associated metals such as aluminum seals.

4. Legislatively require recycled content Requiring that all containers be composed of a percentage of post-consumer material reduces the amount of virgin material consumed.

5. Standardize labeling and inform the public The chasing arrows symbol on plastics is an example of an ambiguous and misleading label. Significantly different standardized labels for "recycled," "recyclable," and "made of plastic type X" must be developed.

Apparently, it's difficult to use these nanoclays. So the researchers turned to "high-powered ultrasonics — high-frequency sound waves too high for human hearing – to separate and disperse the platelets."

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.

when a polymer is molten, the chains will act like spaghetti tangled up on a plate. If you try to pull out any one strand of spaghetti, it slides right out with no problem. But when polymers are cold and in the solid state, they act more like a ball of string.

intermolecular forces affect polymers just like small molecules. But with polymers, these forces are greatly compounded. The bigger the molecule, the more molecule there is to exert an intermolecular force. Even when only weak Van der Waals forces are at play, they can be very strong in binding different polymer chains together. This is another reason why polymers can be very strong as materials. Polyethylene, for example is very nonpolar. It only has Van der Waals forces to play with, but it is so strong it's used to make bullet proof vests.

This is a fancy way of saying polymers move more slowly than small molecules do. Imagine you are a first grade teacher, and it's time to go to lunch. Your task is to get your kids from the classroom to the cafeteria, without losing any of them, and to do so with minimal damage to the territory you'll have to cover to get to the cafeteria. Keeping them in line is going to be difficult. Little kids love to run around every which way, jumping and hollering and bouncing this way and that. One way to put a stop to all this chaotic motion is to make all the kids join hands when you're walking them to lunch. This won't be easy rest assured, as there's always going to be a lot of little boys who are too macho to hold the hands of the girls next to them in line, and some who are too insecure in their manhood to hold anyone's hand. But once you get them to do this, their ability to run around is severely limited. Of course, their motion will still be chaotic. The chain of kids will curve and snake this way and that on its way to eat soybean patties disguised as who knows what. But the motion will be a lot slower. You see, if one kid gets a notion to just bolt off in one direction, he or she can't do it because he or she will be bogged down by the weight of all the other kids to which he or she is bound. Sure, the kid can deviate from the straight path, and make a few other kids do so, but the deviation will be far less than you'd bet if the kids weren't all linked together. It's the same way with molecules.

So then how does this make a polymeric material different from a material made of small molecules? This slow speed of motion makes polymers do some very unusual things. For one, if you dissolve a polymer in a solvent, the solution will be a lot more viscous than the pure solvent.

As you can see, there is no order to the arrangement of the atoms. We call materials like this amorphous. This is the glass that is used for telescope lenses and such things. It has very good optical properties, but it's brittle. For everyday uses, we need something tougher. Most glass is made from sand, and when we melt down the sand, we usually add some sodium carbonate. This gives us a tougher glass with a structure that looks like this:

So is this a polymer or not? Usually it isn't considered as such. Why? Some may say it's inorganic, and polymers are usually organic. But there are many inorganic polymers out there. For example, what about polysiloxanes? These linear, and yes, inorganic materials have a structure very similar to glass, and they're considered polymers. Take a look at a polysiloxane:

So glass could be considered a highly crosslinked polysiloxane. But we usually don't think of it that way. Why not? Probably because even in a highly crosslinked system, you could still trace a polymer chain and see where the crosslinks are. But with glass, it'd be tough to do that.

These methods recover some of the value from the plastic. Recycling recovers the raw material, which can then be used to make new plastic products. Incineration recovers the chemical energy, which can be used to produce steam and electricity. Landfilling plastics does neither of these things. The value of landfilled plastic is buried forever.

A recycling plant uses seven steps to turn plastic trash into recycled plastic:

1. Inspection  Workers inspect the plastic trash for contaminants like rock and glass, and for plastics that the plant cannot recycle.  2. Chopping and Washing  The plastic is washed and chopped into flakes. 3. Flotation Tank  If mixed plastics are being recycled, they are sorted in a flotation tank, where some types of plastic sink and others float. 4. Drying  The plastic flakes are dried in a tumble dryer. 5. Melting  The dried flakes are fed into an extruder, where heat and pressure melt the plastic. Different types of plastics melt at different temperatures. 6. Filtering  The molten plastic is forced through a fine screen to remove any contaminants that slipped through the washing process. The molten plastic is then formed into strands. 7. Pelletizing  The strands are cooled in water, then chopped into uniform pellets. Manufacturing companies buy the plastic pellets from recyclers to make new products. Recycled plastics also can be made into flowerpots, lumber, and carpeting.  

Because plastics are made from fossil fuels, you can think of them as another form of stored energy. Pound for pound, plastics contain as much energy as petroleum or natural gas, and much more energy than other types of garbage. This makes plastic an ideal fuel for waste-to-energy plants.

So, should we burn plastics or recycle them? It depends. Sometimes it takes more energy to make a product from recycled plastics than it does to make it from all-new materials. If that’s the case, it makes more sense to burn the plastics at a waste-to-energy plant than to recycle them. Burning plastics can supply an abundant amount of energy, while reducing the cost of waste disposal and saving landfill space.  

A study by Canadian scientist Martin Hocking shows that making a paper cup uses as much petroleum or natural gas as a polystyrene cup. Plus, the paper cup uses wood pulp. The Canadian study said, “The paper cup consumes 12 times as much steam, 36 times as much electricity, and twice as much cooling water as the plastic cup.” And because the paper cup uses more raw materials and energy, it also costs 2.5 times more than the plastic cup.

scientists have figured out two ways to make plastics degrade: biodegradation and photodegradation.

Photodegradable plastics are a different matter. They use no organic additives. They are made with a special type of plastic that breaks down and becomes brittle in the presence of sunlight. Of course, that means photodegradable plastics do not break down when they are covered by leaves or snow, or when they are buried in a landfill. 

To form polypeptides and proteins, amino acids are joined together by peptide bonds (def), in which the amino or NH2 of one amino acid bonds to the carboxyl (acid) or COOH group of another amino acid as shown in (see Fig. 2). Animation showing the formation of a peptide bond.

The actual order of the amino acids in the protein is called its primary structure (def) (see Fig. 3) and is determined by DNA.

it is commonly said that the order of deoxyribonucleotide bases (def) in a gene determines the amino acid sequence of a particular protein. Since certain amino acids can interact with other amino acids in the same protein, this primary structure ultimately determines the final shape and therefore the chemical and physical properties of the protein.

In globular proteins such as enzymes, the long chain of amino acids becomes folded into a three-dimensional functional shape or tertiary structure (def).

In some cases, such as with antibody molecules and hemoglobin, several polypeptides may bond together to form a quaternary structure (def) (see Fig 6).

Number 6 goes on polystyrene (Styrofoam) items such as coffee cups, disposable cutlery, meat trays, packing “peanuts” and insulation. It is widely accepted because it can be reprocessed into many items, including cassette tapes and rigid foam insulation.

Usually imprinted with a number 7 or nothing at all, these plastics are the most difficult to recycle and, as such, are seldom collected or recycled.

Example 1:A carboxylic acid monomer and an amine monomer can join in an amide linkage. As before, a water molecule is removed, and an amide linkage is formed. Notice that an acid group remains on one end of the chain, which can react with another amine monomer. Similarly, an amine group remains on the other end of the chain, which can react with another acid monomer. Thus, monomers can continue to join by amide linkages to form a long chain. Because of the type of bond that links the monomers, this polymer is called a polyamide.

Example 2:A carboxylic acid monomer and an alcohol monomer can join in an ester linkage. A water molecule is removed as the ester linkage is formed. Notice the acid and the alcohol groups that are still available for bonding.

Because the monomers above are all joined by ester linkages, the polymer chain is a polyester. This one is called PET, which stands for poly(ethylene terephthalate). (PET is used to make soft-drink bottles, magnetic tape, and many other plastic products.)

The two types of polymer configurations are cis and trans. These structures can not be changed by physical means (e.g. rotation). The cis configuration arises when substituent groups are on the same side of a carbon-carbon double bond. Trans refers to the substituents on opposite sides of the double bond.

Three distinct structures can be obtained. Isotactic is an arrangement where all substituents are on the same side of the polymer chain. A syndiotactic polymer chain is composed of alternating groups and atactic is a random combination of the groups. The following diagram shows two of the three stereoisomers of polymer chain.

The ability of an atom to rotate this way relative to the atoms which it joins is known as an adjustment of the torsional angle. If the two atoms have other atoms or groups attached to them then configurations which vary in torsional angle are known as conformations.

different conformation may represent different potential energies of the molecule. There several possible generalized conformations: Anti (Trans), Eclipsed (Cis), and Gauche (+ or -). The following animation illustrates the differences between them.

The geometric arrangement of the bonds is not the only way the structure of a polymer can vary. A branched polymer is formed when there are "side chains" attached to a main chain. A simple example of a branched polymer is shown in the following diagram.

One of these types is called "star-branching". Star branching results when a polymerization starts with a single monomer and has branches radially outward from this point. Polymers with a high degree of branching are called dendrimers Often in these molecules, branches themselves have branches.

A separate kind of chain structure arises when more that one type of monomer is involved in the synthesis reaction. These polymers that incorporate more than one kind of monomer into their chain are called copolymers. There are three important types of copolymers. A random copolymer contains a random arrangement of the multiple monomers. A block copolymer contains blocks of monomers of the same type. Finally, a graft copolymer contains a main chain polymer consisting of one type of monomer with branches made up of other monomers. The following diagram displays the different types of copolymers.

In addition to the bonds which hold monomers together in a polymer chain, many polymers form bonds between neighboring chains. These bonds can be formed directly between the neighboring chains, or two chains may bond to a third common molecule.

Polymers with a high enough degree of cross-linking have "memory." When the polymer is stretched, the cross-links prevent the individual chains from sliding past each other. The chains may straighten out, but once the stress is removed they return to their original position and the object returns to its original shape.

In vulcanization, a series of cross-links are introduced into an elastomer to give it strength. This technique is commonly used to strengthen rubber.

Elastomers,or rubbery materials, have a loose cross-linked structure. This type of chain structure causes elastomers to possess memory. Typically, about 1 in 100 molecules are cross-linked

Natural and synthetic rubbers are both common examples of elastomers. Plastics are polymers which, under appropriate conditions of temperature and pressure, can be molded or shaped (such as blowing to form a film). In contrast to elastomers, plastics have a greater stiffness and lack reversible elasticity.

In contrast to amylopectin, which comprises 70 to 90 percent of natural starch, α-amylose is a branching polysaccharide.

Branches occur at every twelve to thirty residues along a chain of α (1→4) linked glucoses. As a result, amylopectin has one reducing end and many nonreducing ends.

Amylopectin and α-amylose are broken down by the enzyme amylase. In animals, salivary α-amylase begins the digestion process in the mouth. Pancreatic α-amylase continues the process in the intestine.

Glycogen is the energy storage carbohydrate in animals. Glycogen is found mainly in the liver (where it is responsible for up to 10 percent of liver mass) and skeletal muscle (1 to 2 percent of skeletal muscle mass)

However, glycogen branches more abundantly than amylopectin, with branches at every eight to twelve residues. As a result, it has many more nonreducing ends. Glycogen is broken down at these nonreducing ends by the enzyme glycogen phosphorylase to release glucose for energy.

The primary structural homopolysaccharides are cellulose and chitin. Cellulose, a major component of plant cell walls, is the most abundant natural polymer on Earth.

Like α-amylose, cellulose is a linear polysaccharide composed entirely of glucose. However, in cellulose the glucose residues occur in β(1→4) linkage rather than α (1→4) (see Figure 1).

In addition, individual cellulose strands can form hydrogen bonds with one another to provide additional strength. Most animals, including humans, lack the enzymes necessary to dissolve α(1→4) linkages and so cannot digest cellulose

The animals that can (such as ruminants) do so via a symbiosis with bacteria that secrete cellulose-degrading enzymes.

The second most abundant polymer on Earth is chitin. Chitin comprises much of the exoskeletons of crustaceans, insects, and spiders, as well as the cell walls of fungi. Structurally, chitin is very similar to cellulose, except that its basic monosaccharide is N-acetylglucosamine