PHYSICO-CHEMICAL PROPERTIES OF NYLONS
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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?.

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PHYSICO-CHEMICAL PROPERTIES OF NYLONS

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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

amino caproic acid

Poly(caprolactam)


Chemical structure:

Nylon 6,6 o PA-66

Hexamethylene diamine + Adipic acid


Chemical structure:

Nylon 6

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).

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 6circa 60°C

Nylon 6,6circa 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).


50-80

Grandezza fisica

Grado di polimerizzazione

Do polymers degrade upon time?

Thus, can the physico-chemical and mechanical properties of polymers decay upon time?

YES


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.


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


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