Download Synthesis and properties of nylons

PHYSICO-CHEMICAL
PROPERTIES OF NYLONS
L. Costa e P. Bracco
University of Torino
How a polymer chain is structured?
. Which is its chemical structure?
. How long is it?
. How polymeric chains are organised in the rope?
. How do mechanical properties vary?
. Why polymers can degrade?
Chemical structure:
Nylon 6 o PA-6
Caprolactam
H2N
amino caproic acid
(CH2 )5
( NH
COOH + H2N
(CH2 )5
CO
Poly(caprolactam)
)n
+
(CH2 )5
H2O
COOH
Chemical structure:
Nylon 6,6 o PA-66
Hexamethylene diamine + Adipic acid
H2N
( NH
(CH2 )6
(CH2 )6
NH2
NH
+ HOOC
CO
(CH2 )4
(CH2 )4
CO
)n
COOH
+ H2O
Chemical structure:
(
( NH
NH
(CH2 )5
(CH2 )6
NH
CO
Nylon 6
)n
CO
(CH2 )4
CO
)n
Nylon 6,6
Same C/H/N/O ratio, but different structures: they are
different materials with different physico-chemical and
mechanical properties
How long a nylon chain is?
Two definitions:
• n = polymerisation degree (number of monomeric
units into a polymer chain).
( NH
(CH2 )5
CO
)n
For nylon fibres ca. 150 units
• Molecular mass
Can we define a scale model?
Rope:
Ø 10,5 mm,
length 7-8 m.
Amorphous
phase
Crystalline phase
Which are the differences between crystalline and
amorphous phases?
Crystalline phase
Nylon 6 crystal
obtained from a 1-4
butanediol solution
Melting temperature
of the crystalline
phase:
Nylon 6
225°C
Nylon 6,6
265°C
Nylon 6 crystal form
Nylon 6,6 crystal form
Fibres are obtained by
drawing.
The chains are oriented.
Which percent of
nylon is crystalline
phase and which
amorphous phase?
50% of crystalline
phase
What happens during molten nylon cooling?
At the crystallisation temperature, only part of the
polymer chains will organise into crystalline structures,
while some will remain in the molten state, below the
melting temperature, with a high mobility. A further
temperature decrease, will drive to a temperature in
which the chains will tie as in the solid state.
Glasses (Polystyrene):
Tg 100°C
Rubber: Tg -40°C
Bottle (LDPE): Tg -70°C
The transition temperature between the rubbery liquid
state and the glassy solid state is named glass transition
temperature (Tg)
At this temperature the mechanical properties of the
polymers undergo dramatic changes
Which is the Tg of nylons?
Dry:
Nylon 6
circa 60°C
Nylon 6,6
circa 80°C
decreasing in humid atmosphere.
Any other difference between amorphous and
crystalline phase?
In the amorphous phase the chains are more spaced and
small molecules (i.e. water, oxygen) can penetrate into the
polymer, moving through the voids (diffusion process).
Do polymers degrade upon time?
Thus, can the physico-chemical and mechanical
properties of polymers decay upon time?
YES
50-80
Grandezza fisica
Grado di polimerizzazione
The most relevant degradation is a
decrease of the polymerisation degree
If the original polymerisation degree was 150,
2 bond scissions will give chains of 50 units.
Two main processes:
Hydrolytic degradation: due to the action of water.
Can be excluded in the conditions of use of a rope.
Photo-oxidative degradation: the combined effects of light
and oxygen can lead to chain scissions.
The UV radiation (especially with wavelengths below 290
nm) has enough energy for breaking bonds omolitically,
giving very reactive radicals.
The radicals react with oxygen starting a
cyclic process.
O2 by air
Reaction 1
CH2
CH
°
CH2
CH2
CH
CH2
O O°
Reaction 2
CH2
CH2
CH
O
CH2
CH2
CH2
OH
Reaction 3
ketones, alcohols, esters and carboxylic acids
If you wish to know more about the polymers, try
polymers on the web:
http://www.psrc.usm.edu/macrog/index.htm
http://www.polial.polito.it/cdc
Reference:
Nylon Plastics Handbook ed. Melvin L. Kohan
Hanser Publishers Munich
Thank you