Transcript Title

Thermosetting resins
John Summerscales
Thermosets - outline of lecture
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phenol-formaldehyde (phenolic resin)
epoxides (generally diglycidyl ethers)
polyurethanes
bismaleimides (BMI)
unsaturated polyesters (UP or UPE)
vinyl esters
methacrylics
Thermosets
• generally supplied as a liquid
• cross-linked (cured) by chemicals (and heat)
heat reduces the instantaneous viscosity
o heat increases the rate of cure
o cure decreases the viscosity over time
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• product is a 3D molecular network
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whereas a thermoplastic is usually a 2D chain
Total Flow (------)
Ease of Flow
Progress of cure
Temperature
Stages of cure
• A-stage: soluble and fusible
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aka. resol in phenolics
• B-stage: may be swollen
but not dissolved by a variety of solvents
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aka resitol in phenolics
• C-stage: rigid, hard, insoluble, infusible
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aka resit in phenolics
Phenolic resins
• first truly synthetic resins to be exploited.
• Butlerov (1859) formaldehyde polymers.
• Adolf Bayer (1872) phenols and aldehydes
reacted to form resinous substances.
• Arthur Smith (1879) first British Patent
(16274) for phenol-aldehyde resins as an
ebonite substitute in electrical insulation.
• Baekeland (1907) controlled and modified
the reaction to produce useful products
Phenolic resins
• can be broadly divided into three groups:
• resol(e) resins
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typically 1 : 1.5-2 phenol : formaldehyde
• novolac resins
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typically 1 : 0.8 phenol : formaldehyde
• Friedel-Crafts polymers
Phenol (C6H5OH)
• naming reactive sites:
OH
orthometaiso-
para-/tere-
Phenolic resins
• Phenol
...and...
formaldehyde
OH
H
H
sites react first
C
O
Phenolic resin: methylolation
• using Φ to represent phenol
• Φ + CH2O  Φ CH2OH
• Φ CH2OH + CH2O  Φ (CH2OH)2
• Φ (CH2OH)2 + CH2O  Φ (CH2OH)3
Resoles
• alkaline catalyst and excess formaldehyde
• methylene bridge formation
may result in the release of water:
• HO~CH2~Φ~CH2~OH + HO~CH2~Φ
 HO~CH2~Φ~CH2~O~CH2~Φ, or
 HO~CH2~Φ~CH2~Φ~CH2~OH
• continued reaction to network
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first product may lose formaldehyde:
• HO~CH2~Φ~CH2~O~CH2~Φ
 HO~CH2~Φ~CH2~Φ + CH2O
Novolacs
• acid catalyst and excess of phenol:
Φ~CH2~OH + Φ  Φ~CH2~Φ + H2O
• further condensation
and methylene (-CH2-)bridge formation
results in fusible and soluble
linear low MW polymers (novolacs):
~Φ~CH2~Φ~CH2~Φ~CH2~Φ~CH2~Φ~
Crosslinking novolacs I
• add paraform or further formaldehyde
• more usually add HMT
hexamethylenetetramine: (CH2)6N4
• 2 (CH3)2Φ  [(CH3)2ΦCH2]2NH
• 3 (CH3)2Φ  [(CH3)2ΦCH2~]3N
Crosslinking novolacs II
• when the benzylamines are
heated at 180-190°C
in the presence of phenol,
ammonia or methylamine are evolved:
• [(CH3)2ΦCH2]2NH
 (CH3)2ΦCH2Φ(CH3)2 + NH3
• [(CH3)2ΦCH2]3N
 (CH3)2ΦCH2Φ(CH3)2 + CH3NH2
Phenolics
• Generally brittle
due to moisture released during curing
• Exceptional FST properties when burning:
low spread of FLAME
o low emission of SMOKE
o low TOXICITY: only CO2 and H2O released
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• Key markets:
underground railways
o mining
o submarines
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Epoxy resins
Epoxy (glycidyl) groups
O
CH2 CH
CH2
CH2(O)CH-CH2-
Epoxy
Glycidyl
NB: bond angles of 60°, rather than 109°28´ of sp3 hybrid:
highly strained,  highly reactive
Epoxy resin
• Epichlorohydrin: CH2(O)CH-CH2-O-Cl
• Bisphenol-A: HOΦ-C(CH3)2-ΦOH
 CH2(O)CH-CH2-OΦ-C(CH3)2-ΦOH + HCl
O
OH
CH2CH-CH2-O-
CH3
-CCH3
OH
O
-O-CH2CH-CH2
Di Glycidyl Ether of Bisphenol-A (DGEBA)
DGEBA
• methylene (-CH2-) and ether (-O-) groups
give free movement of atoms before cure:
less steric hindrance and higher reactivity for
terminal rather than internal epoxy (oxirane)
o terminal epoxy groups mean
crosslink sites are free of mobile chain ends
so higher Tg achieved.
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Cure of epoxy resins
• Reactive site is the 3-atom epoxy ring
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which may yield an hydroxyl group
• Curing agents include:
amines
o amides
o carboxylic acids
o anhydrides: 2 carboxylic acids with water removed
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Epoxy cure temperatures
• low temperature
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ambient to 60°C
• medium temperature
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up to 120°C
• high temperature
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up to 180°C
• pot-life: time from mixing to 1500 mPas
fibres stick to the brush during lamination
o only tow surfaces are wetted.
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Gel time of epoxy resin
Post-cure
• full cure uses 100% of reactive sites
• but constrained movement of polymer chain may
lead to incomplete cure:
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lower glass transition temperature
lower resin density
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fewer bonds/m3  lower moduli and strengths
additional free volume
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easier diffusion of chemicals  reduced durability
• to achieve optimum high-performance composites,
post-cure at higher temp’ shortly after production:
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unreacted sites may become inactive over time.
Tg for high-performance epoxy
• DGEBA/DICY
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di glycidyl ether of bis phenol A
aliphatic dicyandiamide
• TGDDM/DDS
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Tg ~180-190ºC
Tg ~240-260ºC
tetra glycidyl-4,4‘-diamino diphenyl methane
aromatic diamino diphenyl sulphone
 advantages:
– low cure reactivity … long storage times
– strength retention after time at temperature
 disadvantages
– low strain to failure
– high moisture absorption
– poor hot/wet performance
Epoxy (vs polyester) resin
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outstanding adhesion
excellent static and fatigue strengths
outstanding corrosion protection
excellent chemical resistance
excellent weather resistance
very low shrinkage on curing
good toughness
good heat resistance
from AB Strong “Fundamentals of Composites Manufacturing” (1989)
Epoxy (vs polyester) resin
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medium to high cost
relatively difficult to handle
potential toxicity of uncured material
poor appearance after weathering
from AB Strong “Fundamentals of Composites Manufacturing” (1989)
Polyurethanes
primarily for RIM processes
Bismaleimide (imide group)
Note:
O
C
N
delocalisation across
• benzene ring
• both C=O groups
C
O
• p-orbital on N
 stiff molecule
Unsaturated
polyesters
Curing of polyester resin
-A-B-A-B-A-B- plus styrene (ΦCH=CH2) reactive diluent
where B block contains unsaturation
(double bonds) in backbone of polymer chain, leads to 3D network
- A - B - A |
S
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- A - B - A |
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- A - B - A -
B - A |
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B - A |
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B - A -
B |
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B |
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B -
Curing of polyester resin
Unsaturated polyester and
styrene both have double bonds.
Polyester chain top and bottom,
with 3 styrene molecules
between.
A broken bond
= two free radicals.
Assume all double bonds break.
Five double bonds are
now ten free radicals
Free radicals pair-up
to make new bonds
Molecules move closer together
and rotate to form cross-link.
Net shrinkage of system results.
Note two remaining
reactive sites.
Vinyl esters
• epoxy backbone
• addition-type curing
Methacrylics
• methylmethacrylate instead of
styrene as reactive diluent
Summary (key polymers)
Ph-F
Polymern
Curing
Properties
Condensation
Condensation
Low-cost, brittle, FST
Epoxy
Condensation
Ring-opening
UPE
Condensation
Addition
High-cost,
High-performance
Intermediate cost
Balanced performance