PolymersAdvanced Materials Training and Education Center

FUNDAMENTALS OF CHEMISTRY

What is a polymer?

A polymer is a general term covering materials made up of chains made of many linkslike a bracelet or a locomotive

The individual links come together to form a long chain.

Plastic-a material made up of many links to form a chain. Then main chains form a material called plastics

The simplest example is when ethylene molecules react to form a chain called polyethylene -CH2-CH2-CH2-CH2 …

The result is a gallon plastic milk bottle

General Properties - Polymers

Poor conductor of heat compared to metals-can be filled to make thermally conductive

W/meter*KBtu ft/(hr* ft^2 *deg F orcal*cm/sec*cm^2*K Polyester filler in a sleeping bag

Poor conductor of electricity compared to metals Resistance= Ohms conductance=1/ohms

Few are electrically conductive-can be filled to make conductive

Non Magnetic-unfilled

Optically clear enough to be used for eye glasses and contact lenses-polycarbonate (PC)and poly methyl

methacrylate (PMMA)Canopies on jet fighter aircraft

General Properties - Polymers

Polymer

Plastics Adhesives Fibers

Paints and

coatingsSolutions Gels

Electrically

Conducting

Polymer Types

1. Thermoplastic2. Thermoset

Synthesis of Polymers

1. Chain or “addition“2. Polycondensation “step”

General Properties - Polymers

Properties of polymers in solutionProperties of melted polymers Polymer blending

General Properties - Polymers

Mechanical Testing1. Physical

Tensile Compression Flex Impact Hardness Shear Rheology cone and

plate Viscosity

General Properties - Polymers

nical Testing2. Chemical/physical

FTIR X ray Thermal Swelling Surface analysis

Radical polymerization http://www.pslc.ws/macrog/pe.htm

Polyethylenehttp://www.pslc.ws/macrog/radical.htm

Isotactic- all methyl groups are on one side in polypropylene-allowing the chains to pack to form a crystal. Some defects exist and they form the amorphous (non crystalline part of the polymer material)

Crystallinity depends on Chain Structure

Polymers can be purely amorphous or semicrystalline

Polymer crystallization -not like metals or ceramicsPolymers are not 100 % crystallinePolymers can be semi-crystallineSilk is only 60 % by weight crystalline

Amorphous (non Crystalline area) are shown as the loops on topand bottom of the ordered polymer chains

Amorphous area is where defects in the chain are pushed out to. It is like a Junk yard that contains short chain branches, chain ends, damaged chain structure due to UVradiation

Spherulite- 3D sherical polymer crystal structure from melted polymer such as polyethylene as seen in a polarizing light microscope.

Def. of thermoplastic: The non crystalline portion soften and then the crystal portion melts causing the material to flow. The polymer can harden when cooled.This process can be repeated several times. The polymer can reversibly be made soft and hard by heating and cooling the polymer.

This property is taken advantage of when one wants to make a molded plastic toy Common thermoplastics are typically 100 % amorphous:polyacrylonitrile, polystyrene, polymethyl methacrylate, and poly vinyl Chloride (PVC) are non crystalline, and clear unless pigmented (filled with carbon black or TiO2).Semicrystalline Thermoplastics:Polyethylene, polypropylene, Nylon and and other partially crystalline polymers.Not Cross-linked

Polymer Types - Thermoplastic

ThermoplasticsAbove a critical temperature, called the Glass Transition Temperature (Tg),the chains can move past each other and the material start to flow.The temperature at which softening is visible is called the heat distortion temperature.

Typically the Heat Distortion Temperature is about 20 Deg. C above the Tg

Remember only the non-crystalline portion of the polymer material exhibits a Tg.Only the crystalline portion has a melting point (Tm)

Order of Softening in PE:1. Amorpous portion starts to move 2. The temperature is increased until the crystalline portion melts

EXAMPLE: Plastic forks and knives near the camp fire soften.Never put water in a poly styrene container in the microwave oven.The reason is water boils at 212 F (100 C) at sea level. Steam is even hotter. The container will soften and loose shape. Why?

Coiled chains

Single unit

Thermoplastic: Above Glass Transition Temperature the chains can move about. At a temperature lower than Glass Transition Temperature the chains slow down and just vibrate. They then are considered glassy. An example is a disposable plastic fork at room temperature.

Properties of thermoplastic-Tg

The other type of polymer material is called a thermoset. Once it is hardened or set they can not be made to re-soften and flow like the original materials.They are not reversible. One example is Epoxy adhesive. Another example is a rubber tire.Epoxy materials are made by combining two components:A single chemical unit or a short chain polymer with

chain end that can react.Plus a

An cross linking agent or chemical that leads to cross linking.

Crosslinks are bridges joining two polymers . Typically they bridge growing polymer chains. An example is the sulfur bridge used to link natural rubber polymer chains. This prevents the tires from loosing shape.

The higher the number of cross links the harder the rubber and the stronger the rubber.

Thermoset type Polymers

Thermoset:

Rubber materials used in TN are more cross- linked than those designed for use in Alaska.

Also high mileage tires are more cross linked than Goodyear Eagle tires designed for sports cars because the ones for sports cars are designed to grip the road for higher speeds.

The collagen and elastin in your skin also cross-links to become less elastic when exposed to UV light. Tanning booths can cause this to occur.

General Properties - Polymers

Addition polymerization

A + B -> AB+C -> ABC +D -> ABCD + E-> ABCDE

-LEFT WITH LONG CHAINS AND SINGLE UNITS

-CAN HAVE A MOLECULAR WEIGHT UP TO ONE MIILLION

-PRODUCE A GREAT DEAL OF HEAT DURING THE RECTIONNEED TO REMOVE THE HEAT

-CAN PROCUCE ALL CHAINS OF NEARLY THE SAME SIZE AND WITH BACKBONES REGULAR ENOUGH TO CRYSTALLIZE

Polycondensation synthesis

Many growing chains adding single units, and other growing chains

In the solution vessel one finds 1 unit, 3 unit, 10 unit, 7 unit, 2 unit, and 20 unit chains all hooking up like two trains coming together

Examples: Nylon (polyamide), polyester (polyethylene terepthalate, PET) , polyurethane

During the reaction small molecules like water are removed to form bonds Example: link up amino acids to form proteins

-C-OH + H-N-C- -> -C-N-C- + HOH II I II O H O

Lower molecular weight compared to addition polymers

Properties of polymer solutions

Good solvents – polymer chains are spread out Poor solvent – polymer chains curled up in a coil

Polymer in Good solvent

Polymer in a poor solvent

Higher resistance to flow for same polymer and for the same amount of polymer as below. Only difference is the solvent.

Lower resistance to flowThan above

Polymer solutions continuedGood solvents are ones that are chemically similar or can bond to the polymer

Ex: Jello dissolves in water because it can form many weak bonds To the protein polymer

Water is not that chemically similar but it is a good solvent

Ex: Polystyrene dissolves in Xylene Why? Both PS and Xylene have benzene rings (see two different models of the rings

Polymer solutions-poor solventsPoor Solvent –are chemically dissimilar

Like dissolves likeNow take the opposite viewUnlike causes polymer to drop to the bottom of the glass

Recall water is a polar covalently bonded molecule-Molecules that like water are called hydrophilicMolecules that hate water are hydrophobic

Give an example of each: sugar likes water, but corn oil does notSUGAR IS HYDROPHILICCORN OIL IS HYDROPHOBIC

POLAR BONDED MOLECULES TEND TO DISSOLVE OTHER POLAR MOLECULES.NON-POLAR MOLECULES (NO O,N,F,S, Cl, Br) TEND TO DISSOLVE OTHERNON-POLAR MOLECULES

Polymers when melted for processing

Need to mold a shape like a football helmet?

Need to have a high impact polymer like Lexan or polycarbonate

Need to 1. melt the polymer to a high viscosity liquid state (molasses)2. Mix the melted polymer with chemicals plasticizers and antioxidants3. Need to inject the polymer into a mold4. Cool and release part from the mold

Use a mold matted to an extruder

                        

                                

Cross-section of a plastic extruder to show the screw        

Hopper-polymer feed

Picture the blue as a mold

Screw Extruder

Polymer is feed intothe screw portion via the Hopper

Polymer is moved forward as it is melted

Polymer is forced into the mold as a liquid with high resistance to flow

Part coats the mold, Cools and is ejected

Polymer in the melted state

Polymer is very viscous due to the long chains all entangled

Melt is at a temperature above the Tg and the melting point of any crystals in the Polymer

As the polymer moves down the screw to the mold, the chains are pulled and the chains begin to line up such that the chains are perpendicular to the mold.

Heat transfer, revolutions of the screw, the length of the chains, mold characteristics,Crystallization temperature, and many other factors are important in achieving a quality product.

Quality control is very important

Mold end

As polymer moves down the screw toward the mold chain entanglementsDisappear

Polymer becomes oriented in the machine direction

New sufficiently long chains to provide an article with good mechanicalproperties

Blending Polymers

Many materials are formed from the blending of two or more different plastics or from reacting different chemical units together

Reacting different units together is called forming a co-polymertypes of copolymers:

Random: AABABABBAABBBBAABABAABBABABBA-one Tg-does not have crystal regions

Block: AAAAAAAAAAAAAAAA-BBBBBBBBBBBBBBBBB-two Tg’s if the A’s separate from B’s-example PS-PMMA block copolymer-one domain my be below Tg and the other above

thermoplastic elastomers

Alternating : ABABABABABABAB

A polymer blend is the mixing of two different polymersWhy?To achieve properties different and superior to each on its own

Miscible (fully mixed)-one Tg-single domain, not two domains-may separate under certain conditions

Most polymers do not mix well and tend to separate into domainsOil and water separate into two domains

Most polymers do not mix when heatedThis is because they are large molecules.Recall that most small molecules (except for gases) dissolve when heated

Sugar and water dissolve when heated

Moderately different polymers need a very good reason to mixThey need to like each other chemically at least enoughto form week bonds

Mechanical Testing1. Physical

Tensile:measures strength (stress)by pulling each end of a sample

psi-pounds per square inch or MPa (Mega Pascals, one million Pascals)or dyne per cm2

modulus of elasticity: modulus=Stress/Strainmeasures stiffness of materials, if high the mat. Is brittleGPa or psi

Compression: reverse of a tensile test, initial length is compared to the shorter length as a result of the compressive load, used to test ceramics

Polymer TypeUltimate Tensile Strength (MPa)

Elongation (%)

Tensile Modulus (GPa)

ABS 40 30 2.3ABS + 30% Glass Fiber 60 2 9

Acetal Copolymer 60 45 2.7

Acetal Copolymer + 30% Glass Fiber

110 3 9.5

Acrylic 70 5 3.2Nylon 6 70 90 1.8Polyamide-Imide 110 6 4.5

Polycarbonate 70 100 2.6Polyethylene, HDPE 15 500 0.8

Polyethylene Terephthalate (PET)

55 125 2.7

Polyimide 85 7 2.5Polyimide + Glass Fiber 150 2 12

Polypropylene 40 100 1.9

Polystyrene 40 7 3

Tensile test results For common Polymers and Composites

Note the composite strength and stiffness(modulus)are the highest values compared tonon composite materials

The figure below, from Quadrant Engineering Plastic Products, shows the test geometry.

Tensile test:ASTM D638:For this test, plastic samples are either machined from stock shapes or injection molded. The tensile testing machine pulls the sample from both ends and measures the force required to pull the specimen apart and how much the sample stretches before breaking.

Tensile testing data

Modulus= stiffness=EE=Stress/elongation (strain)

Ultimate tensile strength

Slope = stiffness=E

Elongation of material at break

Elongation=change in length

Area under curve is the toughness

Charpy impact strength testing-measures impact energy or notch toughness

Energy necessary to break the notched specimen is proportional to the height difference h and h’

Used to compare one sample to another 9ASTM Standard E 23 for metallic materials

Flex test-gives a measurement in tension of the underside of the rectangular SampleThe top of the sample is in compression

To calculate the stress for a flex test using a sample with a rectangular cross section

To calculate the flex stress for a sample with a circular cross section

To calculate the elongation (strain) of a flex test

To calculate the modulus (stiffness) resulting from the flex test

Calculations used in flex testing using the diagram

Shear Testing of Materials

X axis measures the distance the material movedY axis is the load applied to cause the movement

Force

Force

Hardness testing

Mostly for metals and ceramicTypes

VickersBrinellRockwell- most used B and C for metals

Rockwell Hardness Testing-

Ball or cone depending on the material is forced into the material with a minor load (10 Kg)

A major load (60 to 150 Kg) is applied and the depth of the penetration is measured

The difference in depth between the minor and major load is the data collected

HRA . . . . Cemented carbides, thin steel and shallow case hardened steelHRB . . . . Copper alloys, soft steels, aluminium alloys, malleable irons, etc.*HRC . . . . Steel, hard cast irons, case hardened steel and other materials harder than 100 HRB*HRD . . . . Thin steel and medium case hardened steel and pearlitic malleable ironHRE . . . . Cast iron, aluminium and magnesium alloys, bearing metalsHRF . . . . Annealed copper alloys, thin soft sheet metalsHRG . . . . Phosphor bronze, beryllium copper, malleable irons HRH . . . . Aluminium, zinc, leadHRK . . . . }HRL . . . . }HRM . . . .} . . . . Soft bearing metals, plastics and other very soft materials**

*Most commonly used** Used for plastics

Scale IndenterMinor Load

F0kgf

Major LoadF1kgf

Total LoadF

kgfValue of

E

A Diamond cone 10 50 60 100

B 1/16" steel ball 10 90 100 130

C Diamond cone 10 140 150 100

D Diamond cone 10 90 100 100

E 1/8" steel ball 10 90 100 130

F 1/16" steel ball 10 50 60 130

G 1/16" steel ball 10 140 150 130

H 1/8" steel ball 10 50 60 130

K 1/8" steel ball 10 140 150 130

L 1/4" steel ball 10 50 60 130

M 1/4" steel ball 10 90 100 130

P 1/4" steel ball 10 140 150 130

R 1/2" steel ball 10 50 60 130

S 1/2" steel ball 10 90 100 130

V 1/2" steel ball 10 140 150 130

Rockwell Hardness Scales

Viscosity-rheology testing of polymer solutions and polymer that are melted

Polymers thin out when shearedin Ketchup behavior observed in an extruder

Polymers have more resistance to flow when 1. the chains are long 2. the temperature is low3. when the same molecular weight molecule is in a good solvent

Polymer electrical properties

Usually used as insulators for electronic devicesEX: polyimide used to insulate wiring in airplanesEpoxy used as an insulator in printed circuit boards like in the Sony Camcorder

Some polymers are electrically conductive when dopedThey are being considered for use in light emitting diode displays

polypyrrolepolydiacetylenepolyanalinepolythiophene

Applicationsflexible displaysflexible batteries

Table of conductivities of materials

Polymer Review What is a thermoset plastic? Is Polyacrylonitrile a

thermoplastic or thermoset Why are polymers good insulator materials? What happens when a thermoplastic reaches Tg? What is needed for two different polymers to mix? What makes plastics mechanical properties unique

compared to metals? If you were going to design a football helmet what

plastic would you use? Would a lightly crosslinked polymer swell in a good

solvent? Why? Explain in two sentences

Exercises

1. Polymer solution and polymer blend using people and string2. Convert units of Force: pounds to Newtons (1 pound=4.448 Newtons)

assuming the force applied is 50 lbs.3. Converts units of stress: psi to MPa (1 psi=6.90X10^-3 MPa) assuming a

stress of a stress of 10^6 psi4. Convert temperature in Kelvin to temperature in centigrade given a

temperature of 273 K (Temp. in Kelvin=273+ Temp. in C)

5. Convert temperature in F to a temperature in C given a temperature of 212 F (Temp. in F= [9/5 (Temp. in C)] + 32

Draw the structural formulafor the polymers below and identify the functional groups in each

PolyacrylonitrileNylonPolyethylene terephthalate (PET)Poly acrylic acidPolyvinyl alcoholPolymethyl methacrylateNeoprene (polychloroprene)

Activities

1. Circulate material and polymer samples-have students classify materials as organic or inorganic, plastic metal or ceramic, and if they are magnetic

2. Have students make a histogram in Excel comparing tensile strengths of HDPE, polystyrene (PS), Nylon 6,6 and polyacrylonitrile (go to MATBASE or matweb)

3. Demonstrate the difficulty in forming a polymer blend vs. a solution of small molecules. People in groups move freely. Give two groups each a string and have them try to move about. Strings have low entropy.

4. Find ASTM standards for tensile of polymer fiber filled composites, particle filled composites and nylon fiber

5. Show youtube video of impact testinghttp://www.youtube.com/watch?v=a_aOlh6dSA8&feature=related

Carbon Fibers Capstone

Chemistry acylonitrilepolyacrlonitrile

ProcessingSpinning fibersConversion to carbon fibers

Surface TreatmentWetting behavior requiresOxidation of CF surfaceOther ways to improve adhesion

Industrial HealthHCNCOAmmonia

Carbon FibersCarbon fiber...the wonder polymer...stronger than steel, and much lighter...but how does one make it? It's made something like this: We start off with another polymer, one called polyacrylonitrile. We take this polymer, and heat it up. We're not sure just exactly what happens when we do this, but we do know that the end result is carbon fiber. We think the reaction happens something like this: when we heat the polyacrylonitrile, the heat causes the cyano repeat units to form cycles!

Chemistry of PAN Carbon Fibers

Then...guess what?...we heat it...AGAIN! Slow roasting the polymer some more at around 400-600 oC causes adjacent chains to join together like this:

This expels hydrogen gas, and gives us a ribbon-like fused ring polymer. But don't think we're done yet! Next we crank up the heat, anywhere from 600 all the way up to 1300 oC. When this happens, our newly formed ribbons will themselves join together to form even wider ribbons like this:

When this happens, we expel nitrogen gas. As you can see on the polymer we get, it has nitrogen atoms along its edges, and these new wide ribbons can then merge to form even wider ribbons. As this happens, more and more nitrogen is expelled. When we're through, the ribbons are really wide, and most of the nitrogen is gone, leaving us with ribbons that are almost pure carbon in the graphite form. That's why we call these things carbon fibers.

PAN Carbon fibers (CF) are-about 90 % carbon-have crystalline and noncrystalline regions-look different from fibers made from pitch

Manufacture of Carbon Fibers from PAN and Pitch

PAN Fiber Manufacture

Air, Ammonia, Propylene reacted by SOHIO Process

Acrylonitrile

Polyacrylonitrile

Polyacrylonitrile + plastisizer (5% methyl acrylate)

Wet spin fibers from solution of polyacrylonitrile

Fibers stretched 2.5 X original length in a coagulation bath

Fibers stretched 14 X in steam at 100 C following washing

Addition polymerization in solution

Mix plastisizer with polymer

Process to Make Acrylonitrile for Polyacrylonitrile

Polyacrylonitrile is a acrylate polymer made by either radical or ionic addition Polymerization-

recall that in the chain reaction A reacts with A reacts with A ……AAAAAAAAAAAAAAAAAAAAAAAAA* + A ->AAAAAAA-(A)-AAAAAAAA

Growing end

Synthesis of Polyacrylonitrile Polymer

Properties of polyacrylonitrile

-primarily isotactic structure >like R in the figure all the CN (nitrile) groups are on one particular side> very regular for packing into a crystal> grows spherulite crystals like other common crystalline polymers

-however due to the polar CN groups an irregular helix results-soluble in a large number of solvents:Dimethylformamide,dimethyl sulfoxide, propylene

Properties of polyacrylonitrile

-Decomposes near melting PT. Can not process from the melted stateMP and Decomposition pt are about 326 Cwater lowers the MP

Thermal Degradation of PANdiscoloration begins

Yellow > red> blackMechanism:

breaking of chainscrosslinkingforming ringsadding H

Products of Decompositionacetonitrile>HCN<nitriles AMMONIA

Crystallinity of PAN

% Wt Crystalline

% Wt quisi crystalline

% Wt. Non Crystalline

42 10 48

Properties of polyacrylonitrile

-Decomposes near melting PT. Can not process from the melted stateMP and Decomposition pt are about 326 Cwater lowers the MP

Thermal Degradation of PANdiscoloration begins

Yellow > red> blackMechanism:

breaking of chainscrosslinkingforming ringsadding H

Products of Decompositionacetonitrile>HCN<nitriles AMMONIA

Polymer Crystals-named spherulites

Purpose of stretching fibers>want to orient the crystals to line up in the same

direction of the long axis of the fiber

Long axis of fiber

Long axis of fiber

Desired Orientation of Crystals in the polymer fiber following stretching

Spinning of polyacrylonitrile

The Spinneret The spinnerets used in the production of most manufactured fibers are similar, in principle, to a bathroom shower head. A spinneret may have from one to several hundred holes. The tiny openings are very sensitive to impurities and corrosion. The liquid feeding them must be carefully filtered (not an easy task with very viscous materials) and, in some cases, the spinneret must be made from very expensive, corrosion-resistant metals. Maintenance is also critical, and spinnerets must be removed and cleaned on a regular basis to prevent clogging.As the filaments emerge from the holes in the spinneret, the liquid polymer is converted first to a rubbery state and then solidified. This process of extrusion and solidification of endless filaments is called spinning, not to be confused with the textile operation of the same name, where short pieces of staple fiber are twisted into yarn. There are four methods of spinning filaments of manufactured fibers: wet, dry, melt, and gel spinning.

Fibers are spun into a Tow

A Tow is a fiber made up of several thousand filaments or single fibers

Spinneret opening provides fibers with a cross section of the following Shapes: Kidney bean, dog bone and cauliflower

Carbon fiber specific process

1. Stabalization PAN fibers in air400-750 Fwith tension

2. Carbonization in nitrogen @900-2700 F with tension

3.Graphitization in nitrogen @3600-6000 F with tension

4. Electrolyte bath

5. Wash, add sizing,dry and then put on a spool

Stretching of fiber

Why stretch ?Stretching moves the crystals into the direction of stretch

Stretching gives the desired orientation of the crystals in the fiber

a high percentage of C-C bonds are put in the direction of tension in the final carbon fiber

Tensile strength is improved drastically by orienting the crystals in the fiber

Draw ratio

Definition: (1) A measure of the degree of stretching during the orientation of a fiber or filament, expressed as the ratio of the cross-sectional area of the undrawn material to that of the drawn material. (2) The ratio of the speeds of the first and second pull-roll stands, used to orient the flat polyolefin mono-filament during manufacture. Definition Copyright ©1989 CRC Press LLC. All rights reserved.

Carbon fiber Mechanical Properties resulting from stretching of PAN fibers

CF Process-PAN

Heat Treatment-oxidation stabilization in air

1. Activated the molecule to cyclize (form rings)2. Makes the rings stable so they do not form more rings3. Chains are kept straight and in the direction of the fiber once tensions 4. released

Fibers are under tension during this step to prevent shrinkage

Poor mechanical properties result if shrinkage is not Controlled

Incommplete oxidation can result in defects from blow-outs during likely to occure during carbonization Step (called pyrolysis-no oxygen)

Heat Treatment-oxidation stabilization in air

Stabilizing Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers. Commercially, the stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

Heat Treatment-oxidation stabilization in air

Stabilizing Step: Prevents Blow outs during carbonizationWater and HCN are given off between 160-200 C

Best Conditions for StabilizationControl oxygen content of fiber

time and temperature are critical factors

A fiber is properly stabilized at this step when the oxygen content is 8-12% by wt.

Deterioration of the fiber occurs above 12 % and lower than 8% gives low yield during the carbonization step

Adding plasticizer helps with stretching and improves the rate of stabilization. Why would that be important?

Heating rate to control shrinkage at the final stages is critical: 1 C /min. at 270C. Shrinkage leads to a poor crystal direction in the fiber.

Over oxidation causes week fibers

PAN backbone chemical effect on carbon fiber

Chains of PAN attract each other. In carbon fiber formation it interferes with 1. Development of crystallinity due to lower mobility of the chains2. Causes shrinkage during heat treatment

Disadvantageous to CF manufacturer because it disturbs orientation of the fiber

Carbonizing StepAlso called the /graphitizing stepAlso it is what chemists call a pyrolysis step-degradation without oxygen

Carbonizing Once the fibers are stabilized, they are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures. The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering. As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber. In some processes, two furnaces operating at two different temperatures are used to better control the rate de heating during carbonization.

Wt loss and gases involved during carbonization

-water, HCN,Ammonia,CO,Nitrogen are given off during this step-Industrial safety is important

-50 % of the original weight is lost during this step

-low heating rate early in process so gases do not cause blow outsAt higher temperatures

most gases come out below 1000 C

Early carbonization (400-500 C) OH groups start crosslinking-organizes and brings together sections that have cyclized (formed rings)Dehydrogenation (400-600 C)Denitrogenation (600-1300 C)

Mechanical Properties and carbonization temperature

Draw Ratio

Precursor fiberTensile Strenght (Gpa)

Fiber carb.@ 1000 C

Fiber carb.@ 2500 C

8 0.27 0.92 0.8313 0.59 1.14 1.2214 0.61 1.5 1.78

6% methacrylate addedDraw temp; 100 C in steamRef. 133

Draw rato increase > then TS increasesHigher TS when carbonized to near 2500 C

Best Properties for PAN fibers

1. Stretching in steam2. Stretching or tension during Carbonization step3. Do not over oxidize during stabilization step4. Manufacture in clean room to prevent reduction of fiber properties5. Very high draw ratio caused defects during carbonization process6. Amount of plasticizer is critical

Structure of PAN Carbon Fiber

Fiber structure

http://www.dailymotion.com/video/x4muy0_zoom-into-a-carbon-fiber_tech

http://www.youtube.com/watch?v=J_gI3chGtww

Carbon fiber structure and carbon fiber cello video

Carbon fibers from pitch

Processing Pitch into fibers

Pitch derived from petroleum, asphalt, coal tar, and PVC

High strength fibers derived from liquid crystalline pitch Preparation of the material is expensive

Fibers are spun from the Liquid Crystalline (mesophase) pitch derived from pitchstripped of light hydrocarbon molecules

yield is 47 % for 100% mesophase pitch

Can be spun into fibers at 287 C

Liquid crystalline behavior

LC=mesophaseMesophase pitches are Nematic

Problems spinning mesophase pitch1. Viscosity is high-need to heat and can not use continuous spinning

1. Melt spinning2. Jet spinning3. Centrifugal spinning

2. Since must be spun at temp. 287-400 C gases outgas leading to defects in the Fiber

3. Resulting fibers derived from pitch with isotropic (non mesophase) portion and anisotropic portion

4. Spinneret has openings of 0.3 mm in diameter. For a 10 um diameter fiber the draw ratio is 2.5 m/sec

5. Spinneret opening shapes: H, Y, triangle, sun burst

Spinning Conditions of Pitch CF

Union carbide

Spinning Temp. C

240-330 c

Spinneret diameter (mm)

0.07-0.38

Number of holes

47-1000

Filament diameterum (microns)

8-50

Draw ratio 3-1702

Filament speedm/min

29-226

Mechanical Prop. Comparison

Precusor Density g/cm3 Young’s modulusGPa

PAN 1.74 230Pitch 1.6 41Mesophase pitchHT 2.2 690

Surface Treatment of Carbon Fibers

Want excellent adhesion of resin to the fiber in a composite

Why?

We want the load on the composite to be transferred from the resin to the fiber.

Why?

The fiber is strong and it should be oriented in direction of the load

Surface Treatment

Good-Fibers are in the direction of the tension (load)

Tension is transferred from the resin (orange) to the fiber when there is good adhesion of the fiber to the resin

Surface Treatment

High Interlaminar Shear Strength (ILSS) prevents shear forces fromdelaminating the composite.

Delamination is when the fiber pulls away from the matrix

Ideally we want a high strength fail to occur in the fiber not a low strength fail in the resin

Surface treatment of carbon fibers

Carbon fibers are treated to protect fibers from breakage

This is called sizing

Carbon fibers are exposed to conditions that modify the surface to provide good adhesion of the fiber to the resin. Today resin treatment and sizing serve same purpose and are one in the same

When forming composites it is necessary for the matrix (resin) to adhere to the fiber

It is important for the resin to be compatible with the fiber

CF surface treatment

Want to form a bond between fiber and resin

Need resin to be able to spread over the fibers

Do not want resin to bead up on surface of the fiber

Need to change the surface of the fiber to be compatible with the resin and for the fibers to allow resin to spread over them

Methods of surface oxidation

Oxidation step following carbon fiber formation puts chemical functional groupson the surface of the carbon fiber

Chemical functional groups formed on the surface of the fiber allow polar resins to spread out over the surface (COOH, C-O, C=O, -OH, -NH2)

Epoxy will not spread over a fiber if the fiber has not been treated.

The goal is to oxidize the fiber in an atmosphere in order to put functional groups on the fiber.

.

Surface energy – energy needed to form an area of new surface

For good spreading (wetting) the surface energy of the surface must be greater than the liquid placed on top

For resin on carbon fiber You do not want this Non wetting

Methods of oxidizing CF surface and associated properties

Methods to raise surface energyoxidation of the CF surface

Other methodsUV lightflameacid

For carbon fibers there are dry and wet methods to introduce groups on the carbon fiber and raising the surface free energy

Surface treatment-oxidation Oxidation in Air

disadvantages: need to go to elevated temperature which causes pitting of the fiber surface

Oxidation in an Oxygen rich atmosphereDisadvantage: has to be carried out above 400 C where spontaneous ignition can occur

Liquid oxidation using oxidizing agentsNitric acid, acidic potassium permanganate, sodium hypochlorate (bleach), hydrogen peroxide-Nitric acid is most widely used as of 1990

C=O OH

O-

C=O NH2

C CO

Acid group on fiber is believed to interact with the hardener (diamine)

The other end of the diamine reacts with the epoxy resin

Wetting is improved

Shear Strength (like ILLS) between resin and fiber is very much improved

Epoxy resin part

Nitric Acid Treated Carbon Fiber in contact with a two part epoxy resinEpoxy is composed of two parts: resin and hardener (diamine)

H NHNH

H

Surface treatment -PlasmaPlasma is an ionized gas

The plasma forms highly reactive gas ions which remove absorbed molecules on the surface of the fiber

Prior to treatment, the Carbon Fiber surface has a low surface energy due to the Organic dirt absorbed onto its surface.

Prior to plasma treatment the surface is not wettable to most resins

Recall poor wetting is when water beads up on a freshly waxed car hood.

We want good wetting for the resin to spread over the fiber.

Ideally we want the fiber to have a surface energy higher than the resin

Surface treatment -electrolytic

+ Anode

CarbonFibers

Polymer with carboxyl ic acid groups with a negative charge –the cathode -

When current flows polymer is plated onto the carbon fiberPolymer coated -CF was put in Epoxy resin The polymer was incorporated into the resin

Significant improvement in shear strength at 10 min treatment timewith electrochemical oxidation

Functionalgroup

Functional group

Adhesion promotormolecule

Adhesion promoter

3-Aminopropyl triethoxy silane

Other ways to improve adhesion

1. Increase surface area

2. Grafting on polymer-must be able to be fixed to the CF surface by Covalent or ionic bonds and be compatible with the resin

3. Crystallization on surface where points on fiber start the crystallization of polymer

PEEK resinNylon

4. Etch week surface features on CF which if left behind would leave an Unreliable, weak interface

5. Inter-diffusion of polymers at the interface. Depth of molecular mixing is criticalFor carbon fibers want polymer to entangle fibers and then bond at many sites.6. Wiskerization-growth of single crystals of SiC or TiO2 on the CF surface Large increase in ILSS results

Laminate Processing

Layers of resin coated woven fiber material is put into a mold and then cured.

http://www.youtube.com/watch?v=9_tDQTgdsCg

http://www.youtube.com/watch?v=GuUdGPAWH9c&NR=1

Carbon fiber is woven into many different patterns to provide desired mechanical properties

Laminate materials-Carbon fiber mat used to make prepreg

Laminate processing Prepreg –a sheet of fiber or cloth which has been coated with

resin and hardener. The prepreg may be partially cured (called B-Stage)

Prepreg sheets are stacked and then subjected to pressure andheat to complete curing

Prepreg laminationCF mates impregnated with thermoset resins are stackedMats may be partially cures-B-staged

Each sheet is rotated 45 degrees with respect to the machine direction or some reference pt.

The stack is put into a heated press where pressure is applied at a temperature above Tg of the resin

The reason why we rotate is we want the composite to be strong in all directions.We want isotropic mechanical properties.We do not want anisotropic properties (strong in only one direction)

Rotating the layers as they are stacked improves the consistency of the expansion of the composite material.

We want similar coefficient of thermal expansion in all directions to prevent unbalanced stresses.

Critical fiber length and overlap

In any composite the ends of the fiber do not help in carrying the load.

It is critical that a fiber be long enough so the load is transferred to the fiber and not to its ends Tensile strength and impact strength depend on the critical fiber length

This is called in fiber composites as the critical fiber length

Critical fiber length

Industrial Health and Carbon Fiber Manufacturing

The total manufacturing of Carbon Fibers produces chemicals that should not be handled with extreme care

The following MSDS of the following chemicals should be available to anyone handling the chemicals.

All Material Safety Data Sheets should be in a binder at your work station or Easily accessible on-line.

Ammonia (NH3), Hydrogen cyanide (HCN), Carbon monoxide (CO) are produced by -the SOHIO process to produce acrylonitrile-The oxidative stabilization step and carbonization step to convert PAN to CF

Industrial Health and Carbon Fiber Manufacturing

Ammonia- NYS Department of Health:http://www.health.state.ny.us/environmental/emergency/chemical_terrorism/ammonia_general.htm

Hydrogen Cyanide-OSHAhttp://www.osha.gov/SLTC/healthguidelines/hydrogencyanide/recognition.html

Carbon monoxide (CO) OSHAhttp://www.osha.gov/SLTC/healthguidelines/carbonmonoxide/recognition.html

HCN Industrial Health

HCN ColorlessBluish white liquidbitter almond odor

Precautions

exposure can cause explosion-polymerizes explosivelycontact with most other chemicals is very hazardousfire will release Cyanide (CN)

Exposure:Recommended by OSHA 5 mg/cubic meter as an 8 hr weighted average

HCN Industrial Health

HCN ColorlessBluish white liquidbitter almond odor

ToxicityRapid asphyxiate. Rapid loss of respiration Prevents tissues in the body from utilizing oxygenDeath can occur in minutes

Signs:Weakness, confusion, headache, dizziness, fatique, anxiety, depressed respiration rate, gasping for air

Carbon Monoxide Industrial Health

COColorless gasOdorless

PrecautionsViolent reaction with strong oxidizing agents like ammonium nitrate and oxygen gasBonds with oxygen carrying molecules in your blood tighter than oxygen

Exposure: OSHA limit – 40mg/cubic meter of air

Carbon Monoxide Industrial Health

COColorless gasOdorless

ToxicityCauses hypoxia due to lack of oxygen to organs and tissues

Signs: Headache, Nausea, weakness, irritability. Those with heart disease experienced leg and chest pain.

Actions to take for exposureRemove individual from site of exposure immediately

Follow instructions in the MSDS

MSDS may change. This is why OSHA requests you to follow MSDS

Have MSDS hard copy in your reach. Update any hard copy versions at least every two weeks if you work in an area of potential exposure.

Both CO and HCN are lethal and not easily detected

Wear protective clothing and breathing apparatus if required by employer.

Ammonia –Industrial Health

Ammonia is produced in the human body through the metabolism of proteins

It occurs from decaying animals and plants

If you buy fish from the grocery store and it smells like ammonia do not buy or eat it.

Ammonia is not as lethal as CO and HCN

It can be lethal in large enough quantities

Never mix bleach with ammonia-It is extremely poisonous

Ammonia (NH3)Forms ammonium hydroxide base in water

colorlesssharp odorlighter than air-will rise

Ammonia –Industrial Health

Ammonia (NH3)Forms ammonium hydroxide base in water

colorlesssharp odorlighter than air-will rise

Precautions

Exposure to vaporscombines with water vapor to form a basebase is a potent irritant base will cause burns –especially mucous membranesCorrosive to cells of the body

Ammonia –Industrial Health

Ammonia (NH3)Forms ammonium hydroxide base in water

colorlesssharp odorlighter than air-will rise

ToxicityBlindnesspermanent lung damage

Signsburning of eyesburning of nose and throatcoughing

Actionmove away from the arearinse area with an excess of water-more than you would think of usingremove exposed close-no time to be modest-

Industrial Health

Other chemicals to read and know msds

Styrene monomer

Acrylonitrile momoner

Nitric Acid HNO3

Hardener-Diamine compound in epoxy (initiator of rxn)

Polyurethane- (Isocyanates)

Argon

Quiz Questions

1. Why do composite fibers need to be a particular length ?2. Why do you need to rotate prepreg mates?3. Why is CO so dangerous in the work place4. How are the ultimate mechanical properties achieved for PAN carbon fibers?5. What parameters must be quality controlled to yield good PAN fibers ?6. Why are clean room conditions important?7. Where should msds information be stored?8. What damage can ammonia cause to the body?9. What are the results to a composite material if the fiber surface is not treated?10.What is ILSS? Why is it important? Remember Peter’s talk11. Why are the intermolecular attractive forces of acrylonitrile important?12.What happens if you over oxidize a fiber in the stabilization step?13.What safety equipment do you need to handle prepreg lay up?14.What is necessary for two different polymers to mix and not separate?15. How can you clean the surface of a carbon fiber so it is wettable?16.What does it mean for a surface to be wettable?

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