Anik Yusuf
Department of Textile Engineering
Ahsanullah University of Science & Technology, Dhaka
Email: anik.yusuf@gmail.com
Introduction:
Polyamides are polymers that contain an amide group (-CONH-) in it’s backbone as a recurring part of the chain. They are frequently referred to as Nylon which has thermoplastic silky properties.
Nylon:
Nylon was the first synthetic fiber produced in 1935 in USA. When less than 85% of amide linkage are attached directly to two aliphatic groups the polyamides are known as Nylon. Nylon fibers are made up of linear macromolecules whose structural units are linked by the amide (–NH–CO–) group. Therefore, these fibers are termed as the polyamides. The most common way for the production of nylon polymers is by the condensation of diamines with diacids.
Fig: Nylon fiber
Invention:
Nylon was the first commercially successful synthetic polymer. Wallace Carothers is credited with the invention of synthetic rubber and nylon around 1933 at DuPont. It was first produced on February 28, 1935 at the DuPont Experimental Station. Fiber went commercial around 1938 and is still used extensively today. DuPont recouped all investment in Nylon 6,6 within 30 days of plant startup as there had been nothing like it before !!!
Nomenclature:
Nylons are formed mainly by condensation polymerization though they can also be formed by addition polymerization .
It can be formed by 3 approaches –
Nylon 6,6:
It’s made from Adipic acid and Hexamethylene diamine. Both of these monomers contain 6 carbon atoms hence the name Nylon 6,6 . Before the name Nylon 6,6 was used, it was called by a codename “fiber 66”.
Nylon 6:
In 1938 Paul Schlack of the IG Farben Company in Germany polymerized Caprolactum & created a different form of the polymer identified as Nylon 6. It’s a homopolymer which is formed by ring opening polymerisation.
***The Degree of Polymerisaton for Nylon 6 & Nylon 6,6 ranges between 100 to 180 & their Average Molecular Weight ranges between 15,000 to 30,000
Spinning Process Diagram:
Fig: Spinning process of nylon
Chemical Properties of Nylon:
(1) Swelling of Nylon:
The Oxygen of the Carbonyl group is slightly negative & the hydrogen (imino hydrogen) is slightly of positive charge. The polar group in Nylon and is responsible for swelling in water or in polar solvents or in dyeing with disperse & metallized dyes.
(2) Melting of Nylon:
Melting point increases because of the following two reasons –
(a) The increase of CONH group to CH2 groups:
The amide group is planar in nature because of the partial double bond character of C-N bond. It has been estimated that the barrier for rotation about this bond may be 63 Kj/mol (15 Kcal/mol) or higher. For this reason if the ratio of CONH group to CH2 is high than melting temperature would be high also.
(b) Hydrogen Bond:
The polar polyamide group is responsible for hydrogen bonding between polyamide chains. Whether the no. of CH2 groups between CONH is odd or even is a very important factor.
Figure: Schematic of a Hydrogen bonded sheet of PA 6 with antiparallel (A) & Parallel (B) orientation of amide group.
If the no. of CH2 group between amide groups is odd & the orientation of the chain is anti-parallel/opposed (Fig. A) than it allows complete hydrogen bonding.But if the orientation of the chains are parallel/same the bonding is not complete.
Now the changing from a parallel array to anti-parallel array requires the inverting of the whole chain in case of odd no. but only a segmental lateral movement is needed if the no. of CH2 group is even (Ex. Nylon 6,6).
So the Nylon 6 polymer has lower melting point than Nylon 6,6
***Based on this we can say that the polyamide having odd no. of CH2 group will have lower melting temperature that the similar kind of polyamide having a even no. of CH2 group***
(c) Introduction of Side Chains:
If side chains are introduced into the carbon skeleton then it interferes with the intermolecular forces between the amide groups. It results in reduction of melting point and increases solubility in organic solvents.
(d) Introduction of Aromatic Ring:
when the amide group is connected to aromatic rings it results in chain stiffening and higher melting temperature. This class of polyamides are called aramids which usually degrades without melting so it can’t be melt spun.
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
