Polymers • Substances containing a large number of structural units

joined by the same type of linkage.

• These substances often form into a chain-like structure.

• Polymers in the natural world have been around since the beginning of time.

• Starch, cellulose, and rubber - possess polymeric properties.

• Man-made polymers studied since 1832.

• Today, the polymer industry has grown to be larger than the aluminum, copper and steel industries combined.

WHAT ARE POLYMERS? Tiny molecules strung in long repeating chains form polymers. Why should you care? •Our body is made of them. DNA, the genetic blueprint that defines people and other living things, is a polymer. •The proteins and starches in the foods we eat, the tires on our bikes and cars, the wheels on skateboards and skates. •Surrounded by polymers every day, everywhere we go.

Another great reason to learn about polymers. •Understanding their chemistry enables in wisely using them. •Once familiar with the varieties of polymers that people make, such as plastics, we can recycle many of them and use them again. •That’s good for the environment.

Polymers at Home

What makes all these different?

Each connects with a different kind of human-made polymer that we encounter in our homes every day.

1

2

3

4

• Water-resistant paints and varnishes derive from a family of synthetic polymers called acrylics. You can also paint yourself warm with acrylics: Spun acrylics find their way into fiberfill jackets and bedtime comforters.

• In 1907 Leo Baekeland patented a revolutionary new material.

• Could mold it at high temperatures and it would retain its shape when cooled, could dye it with brilliant colors.

• Baekeland named it “Bakelite” after himself. • Soon everything from telephones and radios to auto

parts, furniture, and jewelry was being made from Bakelite.

•In a cover story on Leo Baekeland in 1924, Time magazine proclaimed that “in a few years [Bakelite] will be embodied in every mechanical facility of modern civilization. From the time that a man brushes his teeth in the morning with a Bakelite handled brush, until the moment he falls back on his Bakelite bed ...all that he touches, sees, uses, will be made of this material of a thousand uses.”

• World War II pushed plastics production into high

gear. Japanese submarines made it impossible for Allied nations such as Great

Britain and the United States to import latex, the basis of most natural rubber, from Asian plantations.

Industrial chemists rose to the challenge, devising economical means of producing synthetic rubber in huge volumes.

They also created new polymers for use in airplanes, ships, and tanks under fire.

• Silk without silkworms? Practically. The plastic nylon replaced the silk in hosiery in 1938.

Many of the airborne troops in World War II floated to earth beneath nylon parachutes.

Other synthetic fibers such as polyester made the fashions of the 1970s possible.

• Natural rubber from latex, made balls that could bounce.

• But it became hard and brittle when it got too cold, a sticky mess when it got too warm.

• In 1839 Charles Goodyear discovered that latex heated with sulphur—or “vulcanized”—would remain elastic at a wide range of temperatures.

• Sulphur made bridges between the long chain polymers in rubber to keep them from sliding past one another or contracting into knots

• Carriages, cars, trucks, and buses have

traveled billions of miles on tires made from vulcanized rubber and synthetic substitutes.

• Polystyrene foam - made into cartons to protect

eggs or into packing “peanuts” to cushion fragile objects for shipping.

• Insulates - as cups and coolers to keep the warm ones warm and the cold ones cold.

Placed behind walls and ceilings in homes, polystyrene foam helps keep the weather outside at bay.

• Chlorofluorocarbons (CFCs), containing both chlorine and fluorine, were sometimes used to make foam products. This was found to damage the earth’s protective ozone layer-hence phased out their use in the creation of foam packaging and most other types of polystyrene foam.

Polymers in Nature

Everything we see in nature-What do all these have in common? They contain polymers!

You can find different plants, animals, and natural objects that make or contain polymers.

You can build a miniature world of your own.

Rosin• Dead wood and pulp from pine trees contain a polymer

called rosin, which is used to make varnish and soap. Violinists rub rosin on the horsehairs in their bows to make them slide smoothly across the strings. Gymnasts and baseball players use rosin to improve their grips.

Animal Horns• Antelope, buffalo, sheep, cattle, and rhinos all have

horns. Unlike a deer’s antlers, made of bone, horns are made of the polymer keratin.

Parts of ours are made of keratin too: -ingredient in our hair and fingernails. Keratin in the outermost layer of our skin makes it waterproof like other mammals, so one doesn’t get waterlogged the moment he dives in the pool.

Range of applications

Far exceeds that of any other class of material available to man.

Extend from

adhesives, coatings, foams, and packaging materials to textile and industrial fibers;

composites, electronic devices, biomedical devices, optical devices, and precursors for many newly developed high-tech ceramics.

Applications

Industry Automobile parts, windshields for fighter planes, pipes, tanks, packing

materials, insulation, wood substitutes, adhesives, matrix for composites, and elastomers

Agriculture and Agribusiness Polymeric materials -in and on soil to improve aeration, provide mulch, and

promote plant growth and health.

Medicine Many biomaterials, - heart valve replacements and blood vessels, are made of

polymers like Dacron, Teflon and polyurethane.

Consumer Science Plastic containers of all shapes and sizes are light weight and economically

less expensive than the more traditional containers. clothing, floor coverings, garbage disposal bags, and packaging.

Sports Playground equipment, various balls, golf clubs, swimming pools, and

protective helmets

Medical:– Therapeutic apheresis -a treatment process that enables substances which cause disease to be safely removed from the blood while it is outside the body.

A Germany-based medical technology company Fresenius Medical Care developed a technology called DALI® (Direct Adsorption of Lipoproteins) especially for the treatment of patients with severe lipometabolic disorders. It enables LDL cholesterol, also known as “bad cholesterol” because of its influence on vascular calcification, to be extracted from the blood.

-An adsorber filled with a special material electrostatically bonds the LDL cholesterol. For the housing of the adsorber system, a fracture-resistant plastic was required. Makrolon® 2458 developed by Polycarbonates Business Unit of Bayer Material Science AG.

This polycarbonate is sufficiently tough and stiff, which protects it from becoming easily damaged in the often hectic everyday hospital environment.

Withstands the pressurized hot-steam sterilization of the DALI® adsorber, where temperatures reach at least 121 °C for over 20 minutes.”

• Polycarbonate adsorber housings

• Robust in everyday hospital use, suitable for hot-steam sterilization

• The housing of the DALI® adsorber system is made from the fracture-resistant polycarbonate Makrolon® 2458. This withstands pressurized hot-steam sterilization, where temperatures reach at least 121 °C for over 20 minutes.                                  

                            

Plastics in the medical technology sector:

In the DALI® treatment, the patient's blood is removed from an arm vein and passed through the adsorber where the LDL cholesterol sticks to the adsorber globules. The cleaned blood reenters the patient's body via another arm vein

• A further benefit of the polycarbonate is its high transparency which allows continuous visual monitoring of the blood treatment by hospital personnel and therefore enhances patient safety. Makrolon® 2458 meets the requirements of the American standard US-Pharmacopeia, Class VI, relating to the biological compatibility of plastics. Like all medical technology products from Bayer MaterialScience, it also complies with international standard ISO 10993-1 regarding the biocompatibility of plastics that are in contact with body fluids and tissue for up to 30 days.

Future Trends• Nature has used biological polymers as the material of

choice, Mankind chose polymeric materials as the choice material.

From the Stone Age, through the Bronze, Iron, and Steel Ages into its current age, the Age of Polymers.

An age in which synthetic polymers are and will be the material of choice.

• Potential for exciting new applications in the foreseeable future.

Areas as: conduction and storage of electricity, heat and light, molecular based information storage and processing, molecular composites, unique separation membranes, revolutionary new forms of food processing and packaging, health, housing, and transportation.

Polymers will play an increasingly important role in all aspects of our life. • The large number of current and future applications of polymeric

materials has created need for persons specifically trained to carry out research and development in Polymer Science and Engineering-

• can expect to achieve both financial reward and personal fulfillment.

Scientific Principles• The field is so vast and the applications so

varied• Important to understand how polymers are made

and used• There are over 60,000 different plastics

knowledge of this important field can truly enrich our appreciation of this wonder material.

• Companies manufacture over 30 million tonnes of plastics each year, spend large sums on R&D, and more efficient recycling methods.

• Some of the scientific principles involved in the production and processing of these fossil fuel derived materials known as polymers are:

Polymerization Reactions

• The chemical reaction in which high molecular mass molecules are formed from monomers is known as POLYMERIZATION

• Two basic types of polymerization:

1. chain-reaction (or addition) polymerization.

2. step-reaction (or condensation) polymerization.

A three step process involving two chemical entities.

The first, known simply as a monomer, can be regarded as one link in a polymer chain. It initially exists as simple units. In nearly all cases, the monomers have at least one carbon-carbon double bond.

• Initiation• Propagation• Termination

Ethylene is one example of a monomer used to make a common polymer

1. Chain-Reaction (addition) Polymerization

• The other chemical reactant is a catalyst. In chain-reaction polymerization, the catalyst can

be a free-radical peroxide added in relatively low concentrations. A free-radical is a chemical component that contains a free electron that forms a covalent bond with an electron on another molecule. The formation of a free radical from an organic peroxide is :

In this chemical reaction, two free radicals have been formed from the one molecule of R2O2.

With the chemical components identified, a look at the polymerization process

• Step 1: Initiation The first step, 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.

• Step 2: Propagation

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.

• This reaction is continuous because the energy in the chemical system is lowered as the chain grows.

• 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.

• Step 3: Termination 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.

This exothermic reaction occurs extremely fast, forming individual chains of polyethylene often in less than 0.1 second. These polymers have relatively high molecular weights. branches or cross-links with other chains also may occur along the main chain.

These termination types are as below.

Other types of termination are also possible.

• 2. Step-Reaction (condensation)Polymerization Another common type of polymerization. This method 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.).

Eg:Formation of Nylon 66, a common polymeric clothing material, involving one each of two monomers, hexamethylene diamine and adipic acid, reacting to form a dimer of Nylon 66.

The polymer could grow in either direction by bonding to another molecule of

hexamethylene diamine or adipic acid, or to another dimer. As the chain grows, the short chain molecules are called oligomers. This reaction process theoretically can continue until no further monomers and reactive end groups are available. The process is relatively slow and can take up to several hours or days. This process breeds linear chains that are strung out without any cross-linking or branching, unless a tri-functional monomer is added.

Polymer Chemical Structure

• The monomers in a polymer can be arranged in a number of different ways.

• 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:

Copolymer configurations

Polymer Physical Structure • Segments of polymer molecules can exist in two

distinct physical structures. 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, but 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 are randomly arranged and entangled. - 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 begin to move. Above Tg, the mobility is sufficient (if no crystals are present) that the polymer can flow as a highly viscous liquid.

The viscosity decreases with increasing temperature and decreasing molecular weight.

There can also be an elastic response if the entanglements cannot align at the rate a force is applied (as in silly putty). This material is then described as visco-elastic.

In a semi-crystalline polymer, molecular flow is prevented by the portions of the molecules in the crystals until the temperature is above Tm. At this point a visco-elastic material forms.

These effects are as in the specific volume versus temperature graph.

Members of the Polymer Family• Separated into two different groups depending on

their behavior when heated. • Polymers with linear molecules are likely to be

thermoplastic.

These are substances that soften upon heating and can be remolded and recycled. They can be semi-crystalline or amorphous.

• The other group of polymers is known as thermosets. These are substances that do not soften under heat and pressure and cannot be remolded or recycled. They must be remachined, used as fillers, or incinerated to remove them from the environment.

Thermoplastics 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.

Thermoplastics will also retain their newly reformed shape after cooling.

Applications of thermoplastics include: parts for common household appliances, bottles, cable insulators, tape, blender and mixer bowls, medical syringes, mugs, textiles, packaging, and insulation.

• Thermosets Have the same Van der Waals bonds that thermoplastics do. Also have a stronger linkage to other chains. Strong covalent bonds chemically hold different chains together in a thermoset

material. The chains directly bonded to each other or bonded through other molecules.

This "cross-linking" between the chains allows the material to resist softening upon heating.

Thermosets must be machined into a new shape if they are to be reused or they can serve as powdered fillers.

Difficult to reform, but have many distinct advantages in engineering design applications including:

• High thermal stability and insulating properties. • High rigidity and dimensional stability. • Resistance to creep and deformation under load. • Light-weight.

• Applications for thermosets include epoxies (glues), automobile body parts, adhesives for plywood and particle board, and as a matrix for composites in boat hulls and tanks.

Introduction Chee 390

Unit Operations in Polymer ProcessingThermoplastic and thermoset melt processes may be broken down into:

• Preshaping• Shaping• Shape

Stabilization

Introduction Chee 390

Unit Operations in Polymer Processing• Preshaping steps:

– Solids handling and conveying: most processes usually involve feed in particulate form

– Plastication: The creation of a polymer melt from a solid feed.

– Mixing: often required to achieve uniform melt temperature or uniform composition in compounds

– Pumping : The plasticated melt must be pressurized and pumped to a shaping device

• Shaping:The polymer melt is forced through the shaping devices to create the desired shape.

The flow behavior (rheology) of polymer melts influences the design of the various shaping devices, the processing conditions and the rate at which the product can be shaped.

• Shape stabilization:– Involves the solidification of the polymer melt in the desired shape, through

heat transfer

Polymer Processing

Five basic processes to form polymer products or parts.

They are: 1. Injection molding, 2. Compression molding, 3. Transfer molding, 4. Blow molding, and 5. Extrusion

Compression molding and transfer molding are used mainly for thermosetting plastics.

Injection molding, extrusion and blow molding are used primarily with thermoplastics.

Injection MoldingCommon process for forming plastics- involves four steps: 1. Powder or pelletized polymer is heated to the liquid state. 2. Under pressure, the liquid polymer is forced into a mold

through an opening, called a sprue. Gates control the flow of material.

3. The pressurized material is held in the mold until it solidifies.

4. The mold is opened and the part removed by ejector pins.

Advantages of injection molding include rapid processing, little waste, and easy automation.

Molded parts include combs, toothbrush bases, pails, pipe fittings, and model airplane parts.

Diagram of injection molding

Introduction Chee 390

Injection Molding • Injection molding is the most important process used to manufacture

plastic products. It is ideally suited to manufacture mass produced parts of complex shapes requiring precise dimensions.

• It is used for numerous products, ranging from boat hulls and lawn chairs, to bottle cups. Car parts, TV and computer housings are injection molded.

• The components of the injection molding machine are the plasticating unit, clamping unit and the mold.

Introduction Chee 390

Injection Molding Cycle

Injection molding involves two basic steps:– Melt generation by a rotating screw

– Forward movement of the screw to fill the mold with melt and to maintain the injected melt under high pressure

Injection molding is a “cyclic” process:• Injection: The polymer is injected into the mold cavity.

• Hold on time: Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage.

• Cooling: The molding cools and solidifies.

• Screw-back: At the same time, the screw retracts and turns, feeding the next shot in towards the front

• Mold opening: Once the part is sufficiently cool, the mold opens and the part is ejected

• The mold closes and clamps in preparation for another cycle.

Introduction Chee 390

Injection Molding Cycle

The total cycle time is: tcycle=tclosing+tcooling+tejection.

Introduction Chee 390

Molding Processes

Molding techniques for polymers involve the formation of three-

dimensional components within hollow molds (or cavities)

Injection Molding

Thermoforming

Compression Molding

Blow Molding

Rotational Molding

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, but the material moves only a short distance to the mold, and does not flow through gates or runners. Only one part is made from each mold.

Introduction Chee 390

Compression Molding• Compression molding is the most common technique for producing

moldings from thermosetting plastics and elastomers.• Products range in size from small plastic electrical moldings and

rubber seals weighing a few grams, up to vehicle body panels and tires.

• A matched pair of metal dies is used to shape a polymer under the action of heat and pressure.

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.

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

Introduction Chee 390

Blow MoldingBlow molding produces hollow articles that do not require a homogeneous thickness distribution.

HDPE, LDPE, PE, PET and PVC are the most common materials used for blow molding. There are three important blow molding techniques:

• Extrusion blow molding• Injection blow molding• Stretch-blow processes

They involve the following stages:– A tubular preform is produced via extrusion or injection molding

– The temperature controlled perform is transferred into a cooled split-mould

– The preform is sealed and inflated to take up the internal contours of the mould

– The molding is allowed to cool and solidify to shape, whilst still under internal pressure

– The pressure is vented, the mold opened and the molding ejected.

Introduction Chee 390

Extrusion Blow molding

In extrusion blow molding, a parison (or tubular profile) is extruded and inflated into a cavity with a specified geometry. The blown article is held inside the cavity until it is sufficiently cool.

Introduction Chee 390

Injection Blow MoldingInjection blow molding begins by injection molding the parison onto a core and into a mold with finished bottle threads. The formed parison has a thickness distribution that leads to reduced thickness variations throughout the container. Before blowing the parison into the cavity, it can be mechanically stretched to orient molecules axially (Stretch blow molding). The subsequent blowing operation introduces tangential orientation. A container with biaxial orientation exhibits higher optical clarity, better mechanical properties and lower permeability.

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. • An extruder has a hopper to feed the polymer and

additives, a barrel with a continuous feed screw, a heating element, and a die holder. An adapter at the end of an extruder blowing air through an orifice into the hot polymer extruded through a ring die produces plastic bags and films.

Introduction Chee 390

The Single Screw Plasticating Extruder

• Regions 1, 2, 3: Handling of particulate solids• Region 3: Melting, pumping and mixing• Region 4: Pumping and mixing • Regions 3+4: Devolatilization (if needed)

Introduction Chee 390

Cast Film Extrusion• In a cast film extrusion process, a thin film is extruded through a slit

onto a chilled, highly polished turning roll, where it is quenched from one side. The speed of the roller controls the draw ratio and final film thickness. The film is then sent to a second roller for cooling on the other side. Finally it passes through a system of rollers and is wound onto a roll.

• Thicker polymer sheets can be manufactured similarly. A sheet is distinguished from a film by its thickness; by definition a sheet has a thickness exceeding 250 m. Otherwise, it is called a film.

Introduction Chee 390

Sheeting DiesOne of the most widely used extrusion dies is the coat-hanger or sheeting die. It is used to extrude plastic sheets. It is formed by the following elements:

• Manifold: evenly distributes the melt to the approach or land region• Approach or land: carries the melt from the manifold to the die lips• Die lips: perform the final shaping of the melt.• The sheet is subsequently pulled (and cooled simultaneously) by a

system of rollers

Introduction Chee 390

Blown Film Extrusion• Film blowing is the most important

method for producing Polyethylene films (about 90% of all PE film produced)

• In film blowing a tubular cross-section is extruded through an annular die (usually a spiral die) and is drawn and inflated until the frost line is reached. The extruded tubular profile passes through one or two air rings to cool the material.

• Most common materials: LDPE, HDPE, LLDPE

Introduction Chee 390

CoextrusionIn coextrusion two or more extruders feed a single die, in which the polymer streams are layered together to form a composite extrudate.

Process Thermoplastic (TP) or Thermoset (TS)

Advantages Disadvantages

Inj TP, TS

It has the most precise control of shape and dimensions, is a

highly automatic process, has fast cycle time, and the

widest choice of materials.

It has high capital cost, is only good for large numbers of parts, and

has large pressures in mold (20,000 psi).

Comp TS

It has lower mold pressures (1000 psi), does minimum

damage to reinforcing fibers (in composites), and large

parts are possible.

It requires more labor, longer cycle than injection molding, has less shape flexibility than injection

molding, and each charge is loaded by hand.

Trans TS It is good for encapsulating metal

parts and electronic circuits.

There is some scrap with every part and each charge is loaded by

hand.

Blow TP

It can make hollow parts (especially bottles),

stretching action improves mechanical properties, has a fast cycle, and is low labor.

It has no direct control over wall thickness, cannot mold small

details with high precision, and requires a polymer with high

melt strength.

Extru TP

It is used for films, wraps, or long continuos parts (ie.

pipes).

It must be cooled below its glass transition temperature to

maintain stability.

Introduction Chee 390

Product Shaping / Secondary Operations

EXTRUSION

Shaping through die

Final Product (pipe, profile)

Preform for other molding processesBlow molding (bottles),Thermoforming (appliance liners)Compression molding (seals)

Secondary operation Fiber spinning (fibers) Cast film (overhead transparencies, Blown film (grocery bags)

Introduction Chee 390

Annular (Tubular) DiesIn a tubular die the polymer melt exits through an annulus. These dies are used to extrude plastic pipes. The melt flows through the annular gap and solidifies at the exit in a cold water bath.

Introduction Chee 390

Profile diesProfiles are all extruded articles having cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (such as flat film or sheet)To produce profiles for windows, doors etc. we need appropriate shaped profile dies. The cross-section of a profile die may be very complicated

Introduction Chee 390

Secondary Shaping

Secondary shaping operations occur immediately after the extrusion

profile emerges from the die. In general they consist of mechanical

stretching or forming of a preformed cylinder, sheet, or membrane.

Examples of common secondary shaping processes include:

• Fiber spinning

• Film Production (cast and blown film)

Introduction Chee 390

Fiber SpinningFiber spinning is used to manufacture synthetic fibers. A filament is continuously extruded through an orifice and stretched to diameters of 100 m and smaller. The molten polymer is first extruded through a filter or “screen pack”, to eliminate small contaminants. It is then extruded through a “spinneret”, a die composed of multiple orifices (it can have 1-10,000 holes). The fibers are then drawn to their final diameter, solidified (in a water bath or by forced convection) and wound-up.

Introduction Chee 390

Fiber Spinning

• Melt spinning technology can be applied to polyamide (Nylon),

polyesters, polyurethanes and polyolefins such as PP and HDPE.

• The drawing and cooling processes determine the morphology and

mechanical properties of the final fiber. For example ultra high

molecular weight HDPE fibers with high degrees of orientation in the

axial direction have extremely high stiffness !!

• Of major concern during fiber spinning are the instabilities that arise

during drawing, such as brittle fracture and draw resonance. Draw

resonance manifests itself as periodic fluctuations that result in

diameter oscillation.

Introduction Chee 390

ThermoformingThermoforming is an important secondary shaping operation for plastic film and sheet. It consists of warming an extruded plastic sheet and forming it into a cavity or over a tool using vacuum, air pressure, and mechanical means. The plastic sheet is heated slightly above the glass transition temperature for amorphous polymers, or slightly below the melting point, for semi-crystalline polymers. It is then shaped into the cavity over the tool by vacuum and frequently by plug-assist.

Introduction Chee 390

Thermoforming

• Thermoforming is used to manufacture refrigerator liners, shower stalls, bathtubs and various automotive parts.

• Amorphous materials are preferred, because they have a wide rubbery temperature range above the glass transition temperature. At these temperatures, the polymer is easily shaped, but still has enough “melt strength” to hold the heated sheet without sagging. Temperatures about 20-100°C above Tg are used.

• Most common materials are Polystyrene (PS), Acrylonitrile-Butadiene-Styrene (ABS), PVC, PMMA and Polycarbonate (PC)

• Recycled polymers Eg: A typical park.

• Recycling gives new life to the things we use.

• It can - conserve valuable resources: landfill space, energy, raw materials.

• But recycling also takes effort. One place to start is looking at the recycling codes on different packages. The numbers and letters by the triangle will help to sort plastics for recycling.

• Get trash to the recycling center as well.

• Community offers curbside recycling. If not, maybe we need to set up a recycling center near us.

• Recycling: Today's Challenge, Tomorrow's Reward

Overview • Consumer waste poses a challenge to everyone. • Waste solid materials can be grouped into the

following categories: • metals - aluminum, steel, etc. • glass- clear, colored, etc. • paper - newsprint, cardboard, etc. • natural polymers- leather, grass, leaves, cotton,

etc. • synthetic polymers - synthetic rubbers,

polyethylene terephthalate, polyvinyl chloride, etc.

• Plastics constitute between 14 and 22% of the volume of solid waste.

• One possible answer to this problem is recycling. • In 1990, 1 to 2% of plastics, 29% of aluminum, 25% of

paper, 7% of glass, and 3% of rubber and steel as post consumer wastes were recycled. Obviously, increasing the amount of plastics recycled would appear to be the answer. However, a major handicap in the reuse of plastics is that reprocessing adds a heat history, degrades properties and makes repeat use for the same application difficult. For example, the 58 gram, 2-liter polyethylene terephthalate (PET) beverage bottle consists of 48 g of PET, the rest being a high density polyethylene (HDPE) cup base, paper label, adhesive, and molded polypropylene (PP) cap. The cup base, label, adhesive and cap are contaminants in the recycling of the PET.

• contaminants issue in plastic recycling, plastic products designed "reuse-friendly". Products made with recyclability as a viable means for disposal. PET for cost effective recycling. plastic beads are being used to remove paint from aircraft employing a "sand blasting" type method. In place of harsh, environmentally unfriendly chemical solvents use.

• Another reason for not discarding plastics is the conservation of energy. The energy value of polyethylene (PE) is 100 % of an equivalent mass of #2 heating oil. Polystyrene (PS) is 75%, while polyvinyl chloride (PVC) and PET are about 50%. With the energy value of a pound of #2 heating oil at 20,000 B.T.U., land filling plastics results in a waste of energy. Some countries, notably Japan, tap into the energy value of plastic and paper with waste-to-energy incinerators.

• Another factor is the economic trend of progressively increasing tipping fees at landfills. As the cost of land filling of solid waste increases, so does the incentive to recycle. When the cost of land filling exceeds the cost of recycling, recycling will be a practical alternative to land filling.

• Tipping fees, the charge to the waste hauler for dumping a load of solid waste, have been increasing regularly. Municipalities have imposed restrictions and/or have banned the startup of new landfills within their boundaries. As an example, 50% of New Jersey's solid waste is shipped out of state for landfill burial.

• These factors led to certain recommendations by the United States Environmental Protection Agency. EPA's recommendations are: source reduction, recycling, thermal reduction (incineration), and land filling. Each of these is not without its problems. Source reduction calls for the redesigning of packaging and/or the use of less, lighter, or more environmentally safe materials. The trade-off could mean reduced food packaging with the possibility of higher food spoilage rates. There would be fewer plastics, but more food in solid waste to be disposed. Whatever disposal method is chosen, the choice is complex. Whatever the costs, the consumer will bear them.

• Today, consumers are using more products and, therefore, producing more solid waste. As time goes by, we find ourselves with less space to put this waste. Eighty percent of all solid waste is buried in landfills. Today there are one third fewer landfills in operation than the 18,500 available a decade ago, making land-filling much more expensive.

• The amount which synthetic polymers contribute to the weight of solid waste will continue to go up as the use of plastics increases

Recycling of Different Plastics PET (Poly Ethylene Terephthalate) • 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. When chemically treated, the recycled product can be converted into raw materials for the production of unsaturated polyester resins. If sufficient energy is used, the recycled product can be depolymerized to ethylene glycol and terephthalic acid and then repolymerized to virgin PET.

HDPE (high density polyethylene) • 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. Virgin HDPE is used in opaque household and industrial containers used to package motor oil, detergent, milk, bleach, and agricultural chemicals.

• 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. Most recycled HDPE is a colored opaque material, that is available in a multitude of tints.

PVC (polyvinyl chloride) • 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. These fumes are carcinogenic, mutagenic, and teratagenic. This is one of the reasons why PVC must be identified and removed from any plastic waste to be recycled.

• .

LDPE (low density polyethylene)

• 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. Their color is not critical, therefore, regrinds go into black, brown, and to some lesser extent, green and yellow bags.

PS (Polystyrene) • 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. Amoco, another large corporation, currently has a method that converts PS waste, including residual food, to an oil that can be re-refined.

• Currently, 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.

• PVC accounts for less than 1% of land fill waste. When PVC is properly recycled, the problems of toxic emissions are minimized. Various recyclers could reclaim PVC without the health problems. Uses for recycled PVC include aquarium tubing, drainage pipe, pipe fittings, floor tile, and nonfood bottles. When PVC is combined with other plastic waste it is used to produce plastic lumber

• A potential use as plastic lumber. Recycled plastic is mixed with wood fibers and processed

into a replacement for lumber. The wood fibers would have become land fill if not reused. The end product is called Biopaste. This is expected to eventually become a multi-million dollar enterprise. R &D continue to improve this product.

• Recycling is a cost effective means of dealing with consumer plastic waste. Research to reduce the cost of recycling needs to continue. Recycling of plastics is not going to reach the level of the recycling programs of paper and some metals until lower cost, automatic methods of recycling are in place. Fortunately, the solutions to these problems are not beyond the scope of our technology or our minds.

Resin Name

Common Uses Examples of Recycled Products

(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

(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

(PVC-V) Clear food packaging,

shampoo bottles Floor mats, pipes, hoses, mud flaps

(LDPE) Bread bags, frozen food

bags, grocery bags Garbage can liners, grocery bags, multi purpose

bags

(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.

(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

SUPER PLASTICS

The substance (classed as an organic semiconductor) consists of snowflake-shaped molecules and can be used in a variety of light- emitting forms from mobile phone displays to food packaging. It will also be possible to use the material to ‘light up’ wallpaper in a variety of colours as an alternative to traditional overhead lighting. The material is also so flexible and durable that it could be applied to clothing in everything from school uniforms to sports gear. Semi conducting plastic can amplify light - making it one thousand times brighter. This work could, in the future, make the internet faster.

The Future • Recycling is a viable alternative to all other means of

dealing with consumer plastic waste.

• In response to the problem of mixed plastic waste, a coding system has been developed and adopted by the plastic industry. The code is a number and letter system. It applies to bottles exceeding 16 ounces and other containers exceeding 8 ounces. The number appears in the 3 bent arrow recycling symbol with the abbreviation of the plastic below the symbol.

• Western European companies, eg:Hoechst and Bayer,

have entered the recyclable plastic market with success. With a high tech approach, they are devising new methods to separate and handle mixed plastics waste.