Synthesis of Nylon 6
Synthesis of Nylon 6:
Nylon 6 can be synthesized by two processes.
* Batch process
* Continuous process
1.1 Batch process:
Nylon 6 synthesis starts from benzene where under pressure and in the presence of nickel (Ni) catalyst, hydrogen gas (H2) is passed through liquid benzene and cyclohexane is form as an end product of this reaction. The end product is then oxidized under pressure and in the presence of cobalt salt as a catalyst by passing air through cyclohexane to yield 2 products cyclohexanone and cyclohexanol. Cyclohexanol can be produced by an alternative route through phenol hydrogenation under 5 atm pressure and 400 K temperature and in the presence of nickel (Ni) catalyst. Then the cyclohexanol is dehydrogenated into cyclohexanone under pressure and 500 K temperature and in the presence of copper oxide catalyst.
Pure cyclohexanone is needed for Nylon 6 production and it is also known as mixed oil or ketone/alcohol (K/A). Cyclohexanone is now converted into Caprolactam through oxime and in the presence of sulfuric acid (oleum). This isomerization process belongs to Beckmann rearrangement wherein the presence of sulfuric acid oxime is converted into an amide. Rearrangement can be affected by zeolite which can be regenerated at the end and zeolite also save sulfuric acid use. Now caprolactam, molecular mass regulator such as ethanoic acid, and water as a catalyst are added in a reaction vessel to produce nylon 6 polymer, and conditions for this reaction are 500 K temperature, under nitrogen, and for about 12 hours. The whole process explains above is a batch process of nylon 6 production.
1.2 Continuous process
Formation of nylon 6 by Continuous process gives high output, process controllability, and consistent product quality. Nylon 6 synthesis in VK (Vereinfacht Kontinuerlich) tube process which is divided into 3 zones. Firstly Caprolactam is heated at the temperature of 100℃. The molecular weight regulator (acids), initiator (water), delustrants, monomer, pigments, and various other additives are accurately measured and pumped to the upper portion of a vertical VK tube reactor. All components are blended homogeneously at the top of the tube where the temperature is about 220–230℃. The reaction of ring-opening is started and a large amount of water is distilled off.
Now the mixture of reaction is moved downward to the second zone of the VK tube and a temperature of 260℃ is maintained at this zone so that quick polymerization is promoted at this zone. High molecular weight polymer and quick conversion of monomer are achieved by are achieved at relatively low temperature in the 3rd zone of a tube. The whole process in 3 zones takes place in 14–20 hours. Finally, at the end of the tube, the end product as nylon 6 melt can be converted into fiber or either converted into chips. On a brief note, the continuous process of nylon 6 polymerization characterized by Caprolactam heating at the 230–260℃ range of temperature, in water presence and time required for complete polymerization is 12 to 24 hours under nitrogen and carbon dioxide protection. Time of reaction is based upon reactor design, a particular process, and requirements of the end product. During the process water which is used as an initiator for the polymerization reaction of nylon 6 is removed after conversion of 92% of the Caprolactam to the polymer.
Morphology:
Caprolactam is a ring structure and the opening of this ring structure promotes polymerization so that Caprolactam polymerization is known as ring-opening polymerization. The end product is tough, contains high tensile strength, and resistant to chemicals and abrasion. Due to these properties nylon, 6 is used in textile, the electrical, and automotive segments for the production of goods. Crystal morphology and chain conformation of nylon are strongly affected by hydrogen bond presence between amino and carbonyl groups of conjoining chains. The case of the effectiveness of hydrogen bonding depends upon the direction of the chain such as chains that run in a similar direction or lies parallel to each other. Nylon 6 has divergent crystal lattice morphology in comparison to nylon 6,6 as in nylon 6 hydrogen bond formation occurs only when alternately chain run antiparallel to one another. The major difference in the morphology of 2 polymers is the no. of CH2-groups present between -C=O-N-H- (amide) linkage. Nylon 6,6 contain six CH2-groups per block while nylon 6 contain five CH2-groups per block.
