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Thermoset Plastics• Polymerization by chemical reaction of two or more monomer resins often at
high temperature, with catalysts or mixing• Product is insoluble, often intractable• Generally not reversible• Product manufacturing by forming shape during reaction• Examples and applications:
– Polyurethanes: Sport boots, convey belt, coatings– Phenolic: Billiard balls, car distributor caps– Epoxy: paint, adhesives, composites– Unaturated polyesters– Vinyl esters– Bismaleimides– Melamines
• Two piece liquids -Vast bulk of mixture formed by the bulk resin), with catalyst. Mixing carried out by the user.
Thermoset resins are processed in various physical forms. High performance resins have very high viscosity, or may be actually solid at room temperature.
Bulk polymer +catalyst
Mix
CureInitiated
CatalysedLiquid
Crosslinked Solid
No solvent- eliminates waste & shrinkage
• One piece liquids -All components pre-mixed by themanufacturer. Cure is initiated by increased temperature, pressure, or exposure to UV light.
Cure Initiatedw/ elevated Temp
Pre-mixed Catalysed Liquid Crosslinked Solid
Pre-mixed Catalysed Liquid
AddHeat
Crosslinked SolidPre-mixed Catalysed Solid
Cure Initiatedw/ elevated Temp
All components pre-mixed by manufacturer, and the solution remains solid at room temperature. Heat is applied to raise the resin above melting temperature. Further heat and pressure are applied to initiate cure.
• One piece Solids -
TYPICAL OF THERMOSET PREPREGS
As compared to TP’s, TS’s are brittle at room temperature, and cannot be reshaped due to the strong cross-linking covalent bonds.
However, their comparative advantages include;
• Higher tensile strength and stiffness• Excellent chemical and solvent resistance
• Good dimensional and thermal stability
• Good creep resistance
• Excellent fatigue properties
• Low viscosities, simplifying physical processing
Common examples of thermoset polymers include glues, paints, and other surface coatings. We will consider laminating resins, those commonly used as composite matrices.
Thermosets
Liquid Monomer(s), Oligomeric Precursors,
Thermoplastic with curable groups
Elastomer
Glassy Thermosetor
Vulcanized Elastomer
Chemical reaction
Molding: Complex shapes-vulcanizing elastomersReaction Injection Molding: RIM: Mixing two monomers or precursors
Two of the most important transitions are gelation and vitrification;
-The resin transforms from a liquid to a rubbery state.
-There is a drastic increase in viscosity.
Gelation:-Point at which covalent bonds begin to connect between linear chains, forming regions of large networks.
-The reaction continues at a significant rate.
Gel: A two-phase structure.
at Gelation:
Liquid
Crosslinkednetworks
Catalysedsolution
Vitrification:-Occurs when the glass transition temperature of the curing resin increases to the current resin temperature.
-The rate of the cure reaction is significantly reduced, as further crosslinking requires diffusion of molecules through the network.
-The final physical phase depends on the temperature the process has been held at.
μ
(Degree of cure)α0 1
GelPoint
For some manufacturing processes, it is important to consider viscosity of the resin as the cure reaction progresses.
μ
Time
Initially the increase will be relatively slow, significantly quickening as the “gel point” is reached.
During the early stages of cure, the pre-polymeric chains are combining, and the average molecular weight of the mixture increasing. The viscosity therefore increases.
Thermoset resins may assume various physical forms, or phases during the cure reaction, depending on the temperature history.
TTT diagram Depicts:
• Important temperatures• Transitions between states
Note: log time scale!
Kind of like a phase diagramfor metals… remember...
Phase Transformations – (Gel and Vitrification)
The influence of temperature and time is best appreciated throughconsultation of the Time-Temperature-Transformation (TTT) diagram.
Tg is the glass transition temperature of the fully crosslinked polymer.
∞
TgGEL is the temperature at above which gelation occurs before vitrification.
Tg0 is the glass transitiontemperature of the unreactedcomponents.
Below Tg0 the catalysed solution will be a glassy solid. The crosslinking reaction can only occur very slowly, by diffusion (months or years). Thermosets used in prepregs are stored below Tg0 .
Between Tg0 and TgGEL, the catalysed solution is in a liquid state. Crosslinking occurs until vitrification, where a transition to a glassy solid is made. The reaction is very slow thereafter.
Between TgGEL and Tg , the liquid catalysed solution gels first, ending the possibility for flow. Crosslinking continues at a good rate until vitrification.
∞
Above Tg , the liquid catalysed solution gels, but will never vitrify. The polymer remains in a rubbery state throughout the curing process. Once temperature is brought below Tg , vitrification occurs.
∞
∞
At vitrification the resin will not necessarily be 100% cured. Someamount of the unreacted components remain, and the reaction willcontinue very slowly from this point.
The full cure line on the TTT diagram denotes when the cross-linking operation is completed.
Some parts are post-cured at a higher temperature, for a significant time, to ensure full cure, and the best properties (long times though) .
Full Cure
T(z)Q& Q& Q&
Tair Tair
Tair TairThis heat must be removed from the part.
The Thermoset Cure Reaction
Not only are these reactions exothermic, but their reaction ratesare also affected strongly by the local temperature of the resin.
Consider the “slab” of thermoset resin curing below:
Thermoset cure reactions are highly exothermic, generating significant heat during the crosslinking process.
Semi-crystalline thermoplastic
Thermoset
Temperature
HDT TgTm Td
HDT TgTd
(Tm)
Hard, stiff Leathery Liquid
Degraded, Char
Degraded
Hard, stiff Semi-rigid
TemperatureTg
E
Lightly Crosslinked
Solid Thermoset PropertiesDue to crosslinking between polymer chains, thermosets are typically stiffer, but more brittle than thermoplastics.
Their resulting stiffness is a function of the degree of crosslinking, and the application temperature.
Highly CrosslinkedWhile a crosslinked thermoset will not melt,
degradation of the polymer will occur above a certain temperature.
• Epoxy resin is made from the2-part kits.
• It’s the basis of composites like fiberglass, carbon fiber composites etc.
• Apart from an excellent glue, it is an important molding compound for rapid prototyping.
• Tensile strength 60 MPa• Stiffness 2.6 GPa• Chemical and corrosion resistant• Low shrinkage• Cures with amines, alcohols (higher temp) and carboxylic acids
(higher still)
Epoxies
OHHO
MeMe
bisphenol A
Cl O(n + 2)(n + 1)
epichlorohydrin
(n + 2) base
OO
MeMe
MeO OO
MeMe
O
n
Epoxy pre-polymer
OO
MeMe
MeO OO
MeMe
O
n
H2N
Me
O
R
NH2x
R = Mex = 1,2 Jeffamine D230x = 4,5 Jeffamine D400x = 32 Jeffamine D2000
OO
MeMe
MeOO
MeMe
OH
n
HO HN
Me
O
R
HN
x
m
Linear Cured Epoxy
catalyst
Epoxy
Epoxy Curing Chemistry
OO
MeMe
MeO OO
MeMe
O
n
H2N
Me
O
R
x
OO
MeMe
MeOO
MeMe
OH
n
HO HN
Me
O
R x
2
Me Me
O
Me
R Me
NH2
R
O
H2N Me
x
x
3
Me Me
O
Me
R Me
HN
R
O
HN Me
x
x
Insoluble Epoxies: Branched Polyamines
Epoxies
Polyester Thermosets (TS) or Unsaturated Polyesters (UP)
• Largest group of thermosets
• Most like to be reinforced with fiberglass
OO
O
O
O
OO
O
O
O
O
O
OO
O
O
O
OO
O
O
O
O
O
Solution of Polyester and styrene
Peroxide
Heat or light
OO
O
O
O
OO
O
O
O
O
O
OO
O
O
O
OO
O
O
O
O
O
Ph
Ph
Crosslinked thermoset
“Casting Resin”
Unsaturated Polysters
HOOH
O
O
OO
OO O
n
OO O
maleic anhydride
HOO
OH
diethylene glycol
fumaric acid
HO OH
propylene glycol
OO
O
On
Better impact resistance
OO OOO O
HO OH
OO
OO O
O O
Om m
m + n
n m
Reduced cross-link density: lower modulus , less brittle
Phthalic anhydride
Vinyl Esters (VE)
H3C CH3
OOO
O
HO
O
H3C CH3
OO
OH HO
O
O
O
O
H3C CH3
OO
OH HO
O
O
O
Operoxide
Heat
Thermoset
H3C CH3
OO
OH HO
OO
Operoxide
Heat
Thermoset
O
O
H3C CH3
OO
OH HO
OO
O
Thermoset
O
O
nn
Intermediate between polyesters & epoxies in performance and cost
Vinyl Esters (VE)
Formaldehyde Resins• Phenolic
• Urea formaldehyde
• Melamine formaldehyde
Phenol-Formaldehyde Resins
OHO
H H
H+
HO OH
Bisphenol-F
OH
OH
HO
Phenolic resin
Residual formaldehyde in cross-linked matrix
Phenol-Formaldehyde Resins
Novolacs are widely used photoresistsBoth of these are reactive thermosets
OHO
H H
H+
OH
excess
OH
HO n
Novolac or Novolak Resin
OHO
H H
H+
OH
excess
OH
HO n
HO
OH
OH Resole Resin
Thermoset Types
• Phenolics
Photomicrograph (10X) of cross-section of rigid phenol-formaldehyde
Polymeric Foams
• Polymers can be combined with a gas
–Forms voids or cells in the polymer
causing the polymer to be very light
–Referred to as cellular, blown,
expanded polymer, foam
•Elastomeric foam- matrix (polymer) is an elastomer or rubber
•Flexible foam- soft plastic matrix, e.g., plasticized PVC (PPVC), LDPE, PU
•Rigid foams- PS, unsaturated polyesters, phenolics, urethanes (PU)
–Type of polymer matrix, thermoplastic or thermoset can form basis for classification
–Amount of gas added reflects the resulting density
•Light foams: density = 0.01 to 0.10 g/cc (1 to 6 lb/ft3)
•Dense foams: density = 0.4 to 0.6 g/cc (25 to 40 lb/ft3)
–Note: water = 1g/cc or 62.3 lb/ft3
Mechanisms for the formation of cellular structure–Aeration or frothing: mechanical agitation is used to incorporate air into liquid resin system (latex, reactive urethane)
–Physical blowing agent: •Add N2 gas into solution or to liquid melt which comes out of solution when pressure is released and forms cells.•Add liquids at room temperature and have low boiling point. The liquids vaporize upon heating or by chemical reaction heat.
–Aliphatic hydrocarbons (pentane), methylene chloride, trichloro-fluoromethane, or freon 11
–Polystyrene: PS or expanded polystyrene foam (EPS)•Made from expandable polystyrene beads which are small spheres of polystyrene (diameter of 0.3 – 2.3 mm) containing 3-7% pentane as physical blowing agent
–Bulk density of beads (with air spaces) is 0.7 g/cc.
•Manufacturing–Beads are pre-expanded with the use of a steam chamber to a bulk density of 0.02-0.05 g/cc.–Beads are cooled and reached equilibrium with air penetrating the cells.–Placed back in steam chamber and molded into final foamed shape.
»Forms basic cellular structure is closed cell type–Large blocks are molded which are cut into insulating boards or molded into custom products
»Cups, insulating containers, protective elements–Extrusion process can be used with blowing agent
»Meat trays, egg cartons
Chemical Blowing AgentsCompounds that decompose under heat and liberate large amounts of and inert gas,
•N2, CO2, CO, water, ammonia, H2, etc.•Activators can sometimes be added to allow lower decomposition temperature and release more gas at a lower temperature.•Early blowing agents were
–Sodium bicarbonate, which liberates CO2
–Other carbonates and nitrates liberate hydrogen or nitrogen.–Hydrogen can be generated in large quantities, but diffuses away quickly
•Organic compounds can be used for some high temperature thermoplastics
–Toluene sulfonyl hydrazine–azodicarbonamide–Toluene sulfonyl semicarbazide–Phenyl tetrazole
•Can be in finely divided solid form to create cellular structure•Nucleating agents and surfactants are used to control cellular structure
HN
NH
O
H2N S
O
O
HeatNH3 CO2 N2
2NH3 CO2 H2O(NH4)2CO3Heat
NN
O
H2N
O
HeatCONH2 NH4OCN N2
azodicarbonamide
Melamines
N
N
N
NH2 O
H HH2N NH2
N
N
N
NH
HN NH
HO
HO
OH
N
N
N
NH
HN NH
HN
NH
NH
N
N
N
NH2
H2N NH2
H2N CNH2N NH2
O
NN
N
N
N
N
N
NNNH
HN
NH2HN
HN
NH2
Polybismaleimides
R NN
O
O O
O
R
N
N OO
O O
Heat
Heat & peroxides
* *
* *n
Printed wiring boards, carbon fiber composites for aerospaceBrittle but can be toughed with chain extension using Michael addition chemistry
R = O O S S
O
O
O
O
Aliphatics, mixed briding groups
NN
O
O O
O
Heat & peroxidesNN
O
O O
O
Polybismaleimides
Tg > 500 °CTensile Strength 41-83 MPaTensile Modulus 4-5 GPa
Mp 155 °C
Polyimides:
High operating temperatures (up to 500 °C)High tensile strength & modulusLow creep and outgassingFlame resistantSolvent resistant
Composites, high temperature adhesives, wire insulation for extreme environments
H2N R NH2 O O
O
O
O
O
HO2CHN
O
O
CO2H
R
Polyamic acidSoluble & processible
Heat
-2 H2ON N
O
O
O
O
R **
PolyimideInsoluble but really stable
n
OAr
O
O
O
O
O
Thermoplastic Thermoset-like
O
O
O
O
O
OOO
MeMe
H2N NH2
HO2C
O
NH
CO2H
O
OO
MeMe
HN
Polyamic acid
N
O
O
N
O
OO
O
MeMe
Polyimide
Polyimides:Kapton
Kapton( DuPont)
Operating temperature range: -269 °C to 400 °C
Composites, space suits, dielectric material for printed wiring boardsPoor resistance to mechanical wear: wiring in aircraft shorted out due to Kapton failure
Polyimide Thermosets
N N
O
O
O
O
Rn
N N
O
O
O
O
Rn
NN
O
O O
O
N N
O
O
O
O
Rn
NN
O
O O
O
N N
O
O
O
O
Rn
peroxides or azo
peroxides or azos
Polyurethanes
Soft or thermoplastic elastomers
OCN NCO
bis(4-isocyanatophenyl)methane
HOO
OH
Sn(O2CR)2NH
NH
O
O
O
OO
OH*
OCN NCO
H2NO
NH2
Jeffamines
NH
NH
O
NH
O
NH
OOH
*
n
n
x
x
Polyurethane
Polyurea
also MDI (methylene diphenyldiisocyanate)
NCOMe
OCN
Toluene-2,4-diisocyanate (TDI)
OCN NCO
H2NO
NH2
Jeffamines
NH
NH
O
NH
O
NH
OOH
*
nx
x
More flexible Polyureas
H12 MDI
OCNNCO
of
hexamethylene diisocyanate (HMI)
NH N
H
O
NH
O
NH
OOH
*
nx
More Flexible Polyurethanes
Polyurethanes
OCN NCO
NH N
H
O
NH
*
H2N
Me
O
R
x
Me Me
O
Me
R Me
NH2
R
O
H2N Me
x
x
NH
Me
O
R
x
Me Me
O
Me
R Me
NH
R
O
HN Me
x
x
O
Polyurethane or Polyurea Thermosets
Rigid-high moldulus
Polyurethane FoamsOCN NCO
H2NO
NH2
NH
NH
O
NH
O
NH
OOH
*
n
x
x
Polyurea Foam
Pentane (bp 35 °C)
Soft foam
Solvent blown foam
Polyurethane FoamsOCN NCO
H2NO
NH2
NH
NH
O
NH
O
NH
OOH
*
n
x
x
Polyurea Foam
Pentane (bp 35 °C)
Soft foam
Green Polyurethane Foams
H2NO
NH2
Jeffamines
x
OCNNCO
hexamethylene diisocyanate (HMI)NH N
H
O
NH
O
NH
OO1x
OCN
NCOPart A
R
OCN
NCO
1 equivalent H2O
R
OCN
NCO
R
OCN
NH2
+ CO2
Carbon dioxide is generated as blowing agent for foam
R
HN
HN
O
HN
O
HN
A variety of solid properties are relevant to manufacturing;
Polyester
Vinylester
Epoxy
Phenolic
PUR
BMI
PI
Aerospace Al
Carbon Steel
Density
kg/m3
Young’s ModE
GPa
Tensile Strength
MPa
Strain to Failure
%
1.0 – 6.5
3.0 – 8.0
1.5 – 8.0
1.8
400-450
1.5 – 3.3
1.5 – 3.0
1100-1230
1120-1130
1100-1200
1000-1250
1200
1200-1320
1430-1890
2800
7790
3.1 – 4.6
3.1 – 3.3
2.6 – 3.8
3.0 – 4.0
0.7
3.2 – 5.0
3.1 – 4.9
72
205
50 – 75
70 – 81
60 – 85
60 – 80
30 – 40
48 – 110
100 – 110
>540
640
* At room temperature ** Last two materials provided for reference
Common Shaping Processes for Thermosets
Common Thermosets applied to Composites
Resin Type Performance Processing Cost
LOW
HIGH
LOW
HIGHDIFFICULT
EASY• Polyester
• Vinyl Ester
• Polyurethane (PUR)
• Epoxy
• Bismaleimide (BMI)
• Phenolic
• Polyimide (PI).
Additives to Thermoset Polyesters
Fillers like Calcium Carbonate
Tougheners like rubbers
Antioxidants and UV stabilizers
Reinforcements
