PHYSICO-CHEMICAL PROPERTIES OF NYLONS

L. Costa e P. Bracco

University of Torino

. 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?

How a polymer chain is structured?

Chemical structure: Nylon 6 o PA-6

Poly(caprolactam)

n)( CH2 CONH ( )5

CH2( )5COOHH2N CH2( )5

COOHH2N+

H2O+

amino caproic acidCaprolactam

Chemical structure: Nylon 6,6 o PA-66

Hexamethylene diamine + Adipic acid

NH( CH2 CONHn

)CH2 CO( )46 ( )

CH2( )6

NH2H2N CH24

( )HOOC COOH+

+ H2O

Chemical structure:

n)( CH2 CONH ( )5Nylon 6

Nylon 6,6

Same C/H/N/O ratio, but different structures: they are different materials with different physico-chemical and mechanical properties

NH( CH2 CONHn

)CH2 CO( )46 ( )

Can we define a scale model?

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

n)( CH2 CONH ( )5

Rope:

Ø 10,5 mm, length 7-8 m.

Crystalline phase

Amorphous phase

Melting temperature of the crystalline phase:

Nylon 6 225°C

Nylon 6,6 265°C

Crystalline phase

Which are the differences between crystalline and amorphous phases?

Nylon 6 crystal obtained from a 1-4 butanediol solution

Nylon 6 crystal form Nylon 6,6 crystal form

Which percent of nylon is crystalline phase and which

amorphous phase?

50% of crystalline phase

Fibres are obtained by drawing.

The chains are oriented.

tg.dir

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.

What happens during molten nylon cooling?

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

Dry:

Nylon 6 circa 60°C

Nylon 6,6 circa 80°C

decreasing in humid atmosphere.

Which is the Tg of nylons?

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).

Any other difference between amorphous and crystalline phase?

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

Grado di polimerizzazione

50-80 Grandezza fisica

YES

Do polymers degrade upon time?

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.

Photo-oxidative degradation: the combined effects of light and oxygen can lead to chain scissions.

Two main processes:

Hydrolytic degradation: due to the action of water. Can be excluded in the conditions of use of a rope.

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.

ketones, alcohols, esters and carboxylic acids

Reaction 3

°

Reaction 1

Reaction 2

O2 by air

CH2 CH CH2 CH2 CH CH2

O O

CH2 CH2 CH2

CH2 CH CH2

O OH

°

Nylon Plastics Handbook ed. Melvin L. Kohan

Hanser Publishers Munich

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:

Thank you