1. Classes of Polymeric Materials Chapter 3: Thermosets
Professor Joe Greene
CSU, CHICO

2. Thermosetting Resins (thermosets)
Introduction
Thermoplastics are supplied as pellets, powders, or granules and do not undergo a chemical reaction.
Thermoplastics have large molecular weights & long molecules
The high viscosities are reduced by high temperatures
Thermoset resins are supplied as liquid chemicals (low MW and low viscosity) and undergo a chemical reaction that features polymerization and crosslinking.
Liquid chemicals have short chains that polymerized into long chains and high molecular weights and high viscosity.
The chains are crosslinked (attached) to each other to make a stiff molecule
Rubbers involve cross-linking of already polymerized molecules to stiffen the molecules together in Vulcanization
Heat is needed to cause polymerization to build MW and to cause stiffening of molecule through cross-linking
Heat reduces the viscosity of the chemicals until the reaction occurs and then causes the viscosity to get very large during crosslinking.

3. Thermosetting Resins (thermosets)
Types of thermosets
Temperature activated
Catalyst activated
Mixing-activated
Temperature activated Fig 3.84
All thermosets require heat to undergo chemical reaction
Lower temperature thermosets (room temperature cure) react to a more rubbery polymer that gets stiffer upon additional heat.
Pot life: time that it takes for the thermosets to react to a solid after mixed.
Gel time: time it takes for two liquid thermoset polymers that are mixed to form a gel or skin (and stop flowing)
Several thermosets are supplied as powder or granular form.
Heat reduces the viscosity and melts the polymer to allow it to flow & mold
Additional heat triggers a chemical reaction which forms a cross-linked 3D
Common heat activated polymers
Formaldehyde (FOR), phenoplasts (PF), amnioplasts (UF), polyester, vinyl ester, alkyd, allyl, furan, some epoxies, and polyimides

4. Thermosetting Resins (thermosets)
Catalyst activated: Fig 3.85
Some thermosets supplied as stable liquid form
Small amount of liquid (catalyst) is added which starts a chemical reaction and leads to formation of 3D structure.
Chemical type and amount of catalyst controls the extent of reaction and the speed of polymerization.
Many systems can set at room temperature.
Useful for casting resins and for glass fiber reinforced composites.
Common polymer is unsaturated polyester resin (UPR)

5. Thermosetting Resins (thermosets)
Mixing activated systems: Fig 3.86
Some thermosets supplied as two stable liquids.
When the two are added together, a chemical reaction starts and forms a 3D structure.
Ratio of the two chemicals and temperature controls the extent of reaction and the speed of polymerization.
Many systems can set at room temperature.
Useful for casting resins and for glass fiber reinforced composites.
Common polymers are polyurethane and epoxies.
Polyurethane can be mixed at high speeds in a Reaction Injection Molding (RIM) process.

6. Commercial Thermosets
O = C H
Formaldehyde Systems: Functional Groups
Formaldehyde plus one of the three hydrogen containing chemicals to form a 3D molecular network
Phenol,
Melamine, or
Urea.
Condensation reaction involving the oxygen and two hydrogens from two different molecules, Phenol, Urea, or formaldehyde.
One stage systems with resols
Two stage systems with novolacs prepolymers, or precursers
Usually have large amounts of filler, e.g., wood flour, cellulose fibers and minerals.
Supplied as powder or granual form or pills (compacted preforms)
Molding temperatures (125°C – 200°C) and molding pressures of 2000 to 8,000 psi for compression molding and 18,000 psi for injection molding C N
HNH H OH
HN- C- NH O H

7. Commercial Thermosets
Formaldehyde Systems: Functional Groups
Phenoplasts (phenolics) are based on phenol and formaldehyde and were one of the first commercial polymers, Bakelite, and were used for billiard balls.
Used with other materials to act as a binder, adhesive, coatings, surface treatments, etc.
Applications
Temperature resistant insulating parts for appliances (handles, knobs), electrical components (connectors, distributor caps) and bottle closures.
Abrasive binder for grinding wheels and brakes.
Decorative laminates (counter tops or table tops)
Fire resistant rigid foams.

8. Commercial Thermosets
Formaldehyde Systems: Functional Groups
Aminoplasts (amino resins) are based on urea and formaldehyde or melamine and formaldehyde.
Can be made translucent or in light colors for aesthetics
Urea-formaldehyde resins are used for many of the same applications as phenolics if have color requirements
Castable foam system is used for home insulation
Melamine-formaldehyde resins are based on melamine and formaldehyde
Noted for their excellent water resistance.
Used for dishwater safe dinner ware which can be decorated with molded-in paper overlays.
Form the surface layer for decorative laminates (Formica)
Used as an adhesives for water resistant plywood.

9. Commercial Thermosets
C C H
Furan Systems
Feature a ring structure which can be opened cleaved to yield polymeric molecules which have 3-D molecular networks.
Combined with fomaldehyde related thermosets.
Used as binder for sand and foundry work or abrasive particles in grinding wheels.
Used as adhesives and matrix for reinforced plastics where corrosion resistance is important.
Allyl systems (Pg 171)
Manufacture involves the reaction of a monofunctional unsaturated alcohol, allyl alcohol (AA) with a difunctional acid.
Ester linkages are formed though not a polymer
2 unsaturated C=C per monomer permits formation of 3-D molecule with the use of catalysts and elevated temperatures.
DAP (diallylphthalate) is most common allyl monomer
Thermoplastic pre-polymers are available that are cured with little shrinkage
Applications include high performance molding compounds for electrical

10. Commercial Thermosets
Alkyd Systems
Alkyd comes from alcohol (alk) and acid (yd)
Reaction of difunctional alcohol and difunctional acids or anhydrides forms a polyester which is what alkyd is.
Used as coatings (paints, coatings, varnishes)
Unsaturated Polyesters
Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol)
At least some of the acid (or anhydride) features double bonds between adjacent carbon atoms for unsaturation.
Characteristic ester linkages are formed, hence the name Polyester

11. Polyester Chemistry Unsaturated Polyesters
Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol)
C6H4(COOH)2 + (CH2)2(OH) [(CH2)2 -O- C C-O]-
terephthalic acid ethylene glycol Polyethylene terephthalate (PET)
Acids include: maleic, fumaric, isophthalic, terphthalic, adipic, etc.
Anhydrides include: maleic, phthalic
Glycols include ethylene glycol, diethylene glycol, propylene glycol O

12. Polyester Chemistry
Heat or radiation can trigger the cross linking reaction
Catalyst is used
Starts reaction but is not consumed and is retrieved at end of reaction.
Initiator
Methyl ethyl ketone (MEK) peroxide, benzoyl peroxide, and cumene hydroperoxide
Starts reaction, then is consumed in reaction.
Accelerators (or promoters) speed up the reaction.
Inhibitors extend shelf life (hydroquinone, tertiary butyl catechol)
Condensation Reaction results in CO2 and H2O
Monomer required to polymerize, e.g., Styrene, to react with the unsaturations in the polyester molecules to form 3-D network.
Styrene at 30% to 50% in commercial polyester systems for polyester
vinyl toluene for vinyl ester resins
methyl methacrylate

13. Polyester Chemistry
Step 1: Create polymer and build MW of polymer chain
Condensation Polymerization of Di-ACID and Di-ALCOHOL
Fig 2.: Condensation reaction
Connects one end of acid with one end of alcohol to form polyester bond.
The opposite end of acid reacts with another free end of alcohol, and so on .
Have water as a by-product means condensation.
Still have unsaturated polymer. The Carbon atom has double bonds:

14. Polyester Chemistry
Step 2: Crosslink polyester polymer with unsaturated styrene.
Addition (free radical) reaction to connect polyester with styrene
Use a peroxide (free radical) to open the unsaturated bond to form saturation
One reaction starts, the other unsaturated bonds open up and react with the styrene to form a saturated polymer.
The ends of the polyester-styrene crosslinked polymer has peroxide end-groups.
Peroxide is an initiator and not a catalyst since it is consumed in reaction. Catalysts are not consumed in the reaction and can be retrieved at the end of it.

15. Sheet Molding Compound (SMC)
SMC is the paste that is compression molded
33% polyester resin and stryrene, which polymerizes and crosslinks
33% glass fibers (1” fibers)
33% Calcium Carbonate

16. Epoxy Chemistry Epoxy: O H H Other epoxy resins
C C H + H2N (C) N (C) NH2
H H H H
epoxide group amines (DETA) epoxy
Other epoxy resins
diglycidyl ether of bisphenol A (DGEBRA)
tetraglycidyl methylene dianiline (TGMDA
epoxy phenol cresol novolac
cycloaliphatic epoxies (CA)
Curing agents (hardeners, catalysts, cross-linking agents)
aliphatic or aromatic amines (DETA, TETA, hexamethylene tetramine,etc.)
acid anhydrides (phthalic anhydride, pyromellitic dianhydride, etc.)
Active hydrogen react with epoxide groups.
As much as 15% hardener is needed

17. Polyurethane Chemistry
Reaction between isocyanate and alcohol (polyol).
Crosslinking occurs between isocyanate groups (-NCO) and the polyol’s hydroxyl end-groups (-OH)
Thermoplastic PU (TPU) have some crosslinking, but purely by physical means.
These bonds can be broken reversibly by raising the material’s temperature, as in molding or extrusion.
Ratio between the two give a range of properties between a flexible foam (some crosslinking) to a rigid urethane (high degree of crosslinking).
In PUR foams density can range from 1 lb/ft3 to 70 lb/ft3.
Foams are produced by chemical blowing agents.
Catalyst are used to initiate reaction.
RIM process is used to produce fenders and bumper covers

18. Other Thermosets Polyimides Bismaleimide Polybenzimidazoles Phenolics
Carbon Matrices
Thermoplastic matrices
Polyamides
Polypropylene
PEEK
Polysulfone
PPS

19. Polyimides For temperature stability up to 600 F
Polyimides or polybenzimidazole (PBI) rather than epoxy
Aerospace applications due to high cost
Chemical Structure
Polyimides
Characterized by cyclic group containing a nitrogen and two carbonyl groups (C with double bond with oxygen)
PBI
Characterized by a five member ring containing two nitrogens and is attached to a benzene ring.
Polyimids and PBI are structurally planar and very rigid. Large aromatic groups are added into polymer to make stiffer.

20. Polyimides Formed with two step condensation. Fig 2-5
First step: An aromatic dianhydride is reacted with an aromatic diamine to form polyamic (polamide) acid.
Second step: Curing of the polyamic acid.
Formation of imide group by closing of 5-member ring
Condensation step of solvent molecules: water, alcohol, solvents
Chain extension
Cross-linking
High viscosities of polyamid acids require use of prepregs.
Impregnating the fiber mat with monomer solutions of diamines and diester acids.
Long times and gradual increase in temperature are needed.

21. Polyimides Major condensation polyimids, Dupont’s Avimid N & K
are marketed as Prepreg polyimids
Avimid N Tg = 675F (360C), and
Avimid K: Tg = 490F (254C)
Linear polyimids are produced which have thermoplastic behavior above the Tg.
They process like thermoplastics for a few heat cycles.
Advantages of thermoplastic nature
Tractable nature of resins when hot facilitates the removal of volatiles.
Voids, formed as result of the evolution of gases, can be eliminated by applying pressure while heating the resins above Tg.
Applications
Wing skins for high performance aircraft.

22. Polyimides Addition Polyimides
Many polyimids are cross linked with an addition reaction
Two general cross-linking reactions are widely used
End group reactions
Bismaleimide reactions
Reactive End Group Resin Fig 2-6
First phase (imidization): results in the formation of the oligomeric (small polymer) imide
Second phase (consolation): is when the oligomer melts and flows to fill voids that were created from volatiles depart.
Third phase (crosslinking): oligomer builds MW & crosslinks
MW = 1500
Shorter polymer chains gave lower viscosity and better wet-out
Wet-out is defined as uniform coating and soaking of resin in fiber.
Commercial end group resin (PMR) is PMR 11, PMR 15 and PMR 20
PMR-11 has more end groups and higher cross-linking density and higher stiffness
PMR-20 gave better thermal stability.
PMR-15 has the best physical properties balanced.

23. Polyimides
Second type of endgroup crosslinking has acetylene endgroups and is called Thermid 600
Crosslinking
First step: joining two polyimid oligomers to form a butadiene linkage which results in chain extension. Each double bond can react with double or triple bonds to form highly crosslinked.
Addition reaction
Problems is with too fast a cure and chain extension competing with cross-linking mechanism thus causing MW to build too fast.
Alleviated with proper solvents.
Disadvantage is the loss of tackiness in prepregs as the solvent evaporates.

24. Polyimides Bismaleimide (BMI) resins Addition polymerization
Reactions involving bismaleimide (BMI) derivatives: Fig 2-8
Case 1
Carbon-Carbon double bond in the maleimide group reacts with the carbon-carbon double bond in the olefin co-reactant (similar to maleic acid is crosslinked with styrene in polyester)
Case 2
An aromatic diamine adds to the carbon-carbon double bond of the maleimide in what is called Michaels Reaction.
Both cases: the coreactants (olefin or diamine) form bridges between the imide molecules to form a crosslinked structure
Commericial products
Ciba-Geigy uses an olefinic compound with two olefins

25. Polyimides Bismaleimide (BMI) resins Advantages
Low processing temperature versus polyimides (Cured at 350F)
Standard epoxy processing equipment can be used since same T.
Postcure of 475 F is required to complete polymerization.
BMI are fully formed polyimides when reacted to form composite
Thus, no volatiles are removed and no consolidation problems
Tack and drape are quite good because of the liquid component of the reactants

26. Polyimides Polybenzimidazole (PBI) resins Formation reaction- fig 2-9
Less prevalent than the polyimides, PBI have equivalent and sometimes superior physical and thermal properties
Formation reaction- fig 2-9
Five member ring containing two nitrogens is formed with accompanying aromatic groups.
Groups are flat and stiff leading to good physical properties and aromatics result in high thermal.
Problems are expensive, difficult process, toxicity
Some have been alleviated and is commercially available
Resin is thermoplastic with a Tg over 800F (427C)
It does not burn, contribute fuel to flames or produce smoke
Forms a tough char
Resins are toxic and need to be handles with care.

27. Phenolics Phenolics is an old thermoset resin
Used for general purpose, unreinforced plastic
electrical switches
junction boxes
automotive molded parts
consumer appliance parts, handles, billiard balls
Fillers are required due to high shrinkage and brittle nature.
Sawdust, nut shells, talc, or carbon black
Fiber reinforced Phenolics have aerospace applications
Rocket nozzles, nose cones due to ablative nature (Goes from solid to gas during burning)
High temperature aircraft ducts, wings, fins, and muffler repair kits

28. Phenolics Phenolic chemical structure-
Formed by reaction between phenol and formaldehyde
Condensation reaction releases water as a byproduct.
Initially low molecular weight, soluble and fusible, A-Stage resin
Condensation reaction involves more and more phenol molecules that causes the resin to pass through a rubbery, thermoplastic state that is only partially soluble phase called B stage.
Resin is cured and cross-linked thermoset resin, C- Stage.
Other terms describing phenolic formation
Resole: If phenol/formaldehyde reaction is carried out in excess formaldehyde and base catalyst is called resole at low molecular weight stage. Requires just heat to convert to C-stage (1 step)
Novolac: If phenol/formaldehyde reaction is carried out in excess phenol with an acid catalyst is called novolac.
Requires addition of a hardener (hexamethylenen tetramine) to achieve C- Stage in 2 steps. It provides acid to both reactants which speeds up reaction.
Reinforcements are mixed with novolacs for composites. Bstaging is when any other resin is cured to an intermediate stage and cured by heating

29. Thermoplastic Composites
Plastics are reinforced with glass and a few with carbon fiber
Nylon, PP, PBT, PEEK and PEK, and Polysulphone
Advantages
Requires less processing time since it is heated and not cured.
Thermoplastic pre-preg sheets have infinite shelf life versus thermoset
Disadvantages
Have lower thermal resistance than most thermoset composites
Have lower strength and modulus than some thermoset composites
Have difficulty wetting out high fiber loading composites.

30. Thermoplastic Matrices
Two types of thermoplastic composites: Discontinuous and continuous reinforcements
Discontinuous fiber- Conventional thermoplastics and short (3mm) or long fibers (6mm)
Polypropylene, nylon, PET, PBT, Polysulphone, PE, ABS, PC, HIPS, PPO
Short Glass or Carbon fiber increases
Tensile strength, modulus, impact strength, cost, thermal properties
Short Glass or carbon fiber decreases
Elongation,
CLTE,
Moisture
sensitivity

31. Thermoplastic Matrices
Several types of resin types
Conventional plastics: Less expensive (< $2 per pound)
Commodity plastics : PP, PE, PVC, PS, etc. (<$1 per pound)
Engineering resins: PC, PET, PBT, Nylon, ABS etc. (>$1pp)
High Performance Plastics: High Costs (> $10 per pound) and High Thermal Properties
PEEK, PEK, LCP, PPS, Polyaryle Sulfone, Polysulfone, Polyether sulfone, Polyimid
PEEK and PEK = $30 per pound
Polyarylesters
Repeat units feature only aromatic-type groups (phenyl or aryl groups) between ester linkages. Called wholly aromatic polyesters O n C O n C
PolyEther-Ether-Ketone (PEEK)
PolyEther-Ketone (PEK)

32. Properties of Reinforced PEEK

33. Advantages and Disadvantages of Polyketones
High continuous use temperature (480F)
High toughness, especially at high temperatures.
Outstanding wear resistance
Excellent water resistance and better than thermoset composites
Excellent mechanical properties
Very low flammability and smoke generation
Resistant to high levels of gamma radiation
Higher Elongation (30%-100%) versus thermosets (1%-10%)
Disadvantages
High material cost and long processing times
High processing temperatures due to high viscosities (1 Million poise) versus thermoset composites (Epoxy = 10 poise). Syrup = 1000 poise
Moderate or poor resistance to hot oils
Difficult to have high fiber loadings due to high viscosity
Need special processing techniques; comingle plastic powder with fiber sheet and consolidate (impregnate resin in fiber bundle) through heated rollers.

34. Thermoset Reacting Polymers
Process Window
Temperature and pressure must be set to produce chemical reaction without excess flash (too low a viscosity), short shot (too high a viscosity), degradation (too much heat)

35. Compression Molding of Polyesters
Compression molding was specifically developed for replacement of metal components with composite parts.
Materials can be either thermosets (SMC) or thermoplastics (GMT)
Most applications today use thermoset POLYESTER polymers, e.g., SMC or BMC. In fact,compression molding is the most common method of processing thermosets.

36. Resin Transfer Molding of Polyester or Epoxy
In the RTM process, dry (i.e.,unimpregnated ) reinforcement is pre-shaped and oriented into skeleton of the actual part known as the preform which is inserted into a matched die mold.
The heated mold is closed and the liquid resin is injected
The part is cured in mold.
The mold is opened and part is removed from mold.

37. Open Mold Processing of Composites
Open Mold processes of Polyester or Epoxy
Vacuum bag, pressure bag, SCRIMP
Autoclave: Apply Vacuum Pressure and Heat in an oven which can be 5 feet to 300 feet long

38. Polyurethane Processing
Polyurethane can be processed by
Casting, painting, foaming
Reaction Injection Molding (RIM)

39. Structural RIM for Urethanes (Fast RTM)
Fiber preform is placed into mold.
Polyol and Isocyanate liquids are injected into a closed mold and reacted to form a urethane.

40. Composite Reinforcement Classifications
Reinforcement Type
Discontinuous (fibers are chopped and dispersed in matrix resin)
Short fibers: fiber lengths 3mm or less (glass filled plastics, GF-Nylon)
Long fibers: fiber lengths greater than 6 mm. (Some injection molded materials with 6mm fibers, Sheet Molding Compound (SMC) with 1” fibers, DFP Directed Fiber Preforms for RTM and SRIM)
Particulates: fibers is forms as spheres, plates, ellipsoids (some injection molded materials reinforced with mineral fibers)
Continuous (fibers are throughout structure with no break points)
Glass roving: glass bundles are wound up in a packet similar to yarn.
Roving is woven into several weaves using a loom machine like in apparel.
Mat products: random swirl glass pattern.
Woven product: roving is woven into machine direction (warp) and cross direction (weft)
Uni product: roving is woven in one direction with a cross thread given to hold mat together.

41. Processing of Fiber Reinforcements
Carbon fiber or glass fiber
Hand lay-up and Spray-up
Filament winding

42. Injection Molding Glass Reinforced Composites
Plastic pellets with glass fibers are melted in screw, injected into a cold mold, and then ejected.
Glass filled resin pellets

43. Composites Can Have a Fiber Preform
Fiber type
Roving form that can be sprayed into a 3-D preform
Roving form that is woven into a glass sheet and then formed to shape (preform)

44. Glass Fibers
Properties of Glass Fibers: (Table 3-1)

45. Carbon/Graphite Fibers
Need for reinforcement fibers with strength and moduli higher than those of glass fibers has led to development of carbon
Thomas Edison used carbon fibers as a filament for electric light bulb
High modulus carbon fibers first used in the 1950s
Carbon and graphite are based on layered structures of hexagonal rings of carbon
Graphite fibers are carbon fibers that
Have been heat treated to above 3000°F that causes 3 dimensional ordering of the atoms and
Have carbon contents GREATER than 99%
Have tensile modulus of 344 Gpa (50Mpsi)

46. Carbon Fiber Mechanical Properties
Note: 1Mpsi = Mpa

47. Organic Fiber- Kevlar Properties
Properties- Table 3-3
Kevlar has high heat resistance, though less than carbon fiber.
Kevlar has exceptional exposure limits to temperature
No degradation in properties after 7 days at 300 F.
50% reduction in properties after 7 days at 480F.
50% reduction in properties after 12 months of sunlight exposure in Florida
Kevlar are hygroscopic and are susceptible to moisture and need to be dried
Aramids do not bond well to matrices as do glass and carbon fibers
The ILSS (interlaminar Short beam shear) values are lower.