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Elastomers
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Elastomers
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Definition- materials capable of reversible change in
length at operating temperatures, that is, once a load is
removed the material returns to its original dimensions;
a rubbery compound;
Generally amorphous thermosets with Tg below room
temperature to allow full chain mobility- the restoring
force in entropic;
referred to as “memory”
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Types
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Natural and synthetic rubber-Thiokol, Buna, Neoprene
Polyurethanes
Silicone-polysiloxanes
Natural proteins-elastin
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Mechanical Properties
10
Fibre
Rigid
Plastic
5
10
4
Stress
Flexible Plastic
N/cm2
10
B
Elastomer
A
0
10
1
Strain D L/ L
5
6
3
2
Elastomers have properties that can be explained by a
model that shares characteristics of thermosets and
thermoplastics -have all of the domains shown below
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Principles of Polyurethane
Chemistry
High molecular weight polymers based isocyanate
chemistry
Polyisocyanates have the following general formula:
R-(N=C=O)n, n= 2-4
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Addition of Nucleophilic
Reactants
The most important reactions of isocyanates is the
formation of carbamic acid derivatives (7) by the addition
of components with an acidic H-atom (6) across the C=N
double bond (1).
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OH group-containing compounds (8) are by far the
most important reactants for isocyanates. They are
added under mild conditions, forming carbamic esters
(9). Primary alcohols, secondary alcohols, and
phenols show decreasing reactivity in that order.
The trivial name urethane which is used for the
compound ethyl carbamate, gave the whole
polyurethane chemistry its name: polyisocyanates and
polyols form polyurethanes.
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Intermolecular Bonds in
Polymers
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The more nucleophilic primary and secondary amines
react much more vigorously with isocyanates. In this
reaction, ureas are formed (11).
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Important Building Blocks for
Polyurethanes- Isocyanates
OCN
CH2
MDI or aromatic
polyisocyanate
NCO
CH3
CH3
NCO OCN
NCO
TDI-80
NCO
OCN
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20%
80%
CH2
OCN(CH2)6NCO
NCO
H12MDI
HDI
11
Polyols
HO(CH2CH2CH2CH2O)nH
PTMO
HO(CH2CH2O)nH
PEO
CH3
HO(CHCH2O)nH
CH3
HO(CH2)x
Si O
CH3
O
HO
R OC
PPG
CH3
Si (CH2)xOH
y CH3
O
(CH2)x CO
R OH
y
PSX
Synthetic Scheme of
Polyurethanes
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Typical Chain-extenders
HOCH2CH2OH
CH2 OH
HOCH2CHOH
CH OH
CH3
CH2 OH
HO(CH2)4OH
HO(CH2)6OH
H2N(CH2)2-6NH2
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Polyurethane Structural Model
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Schematic Model Domain
Sturcture
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Hard Segment Interactions
affect Mechanical Performance
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Effect of Polyurethane MW on
Mechanical Properties
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