Nylon 6

Nylon 6 is a member of the family of nylons that consists of several different types. Nylon 6/6, nylon 6, nylon 6/10, nylon 6/12, nylon 11, nylon 12, and nylon 6-6/6 copolymer are the most common. Of these, nylon 6/6 and nylon 6 dominate the market. The numbers refer to how many methyl units (-CH2-) occur on each side of the nitrogen atoms (amide groups). The difference in number of methyl units influences the property profiles of the various nylon 6. Moisture absorbance is decreased due to reduced polarity with further separation and less regular location of the very polar amide roups. Resistance to thermal deformation is lowered due to more flexiblity and mobility in these methyl unit sections of the main chain. As these units increase in length, making the molecules appear more like polyethylene, the properties of the nylon 6 shift slightly toward those of polyethylene. Not considering the effects of moisture, Nylon 6/12 has lower modulus, higher elongation, lower strength, lower thermal distortion temperature, lower hardness and lower melting point than nylon 6/6. One relationship which does not conform is price. Nylon 6/12 is more expensive than nylon 6/6. The property which gives nylon 6/12 its utility is moisture absorption which is approximately half of that of nylon 6/6. This means the properties are much more consistent and experience less fluctuation due to ambient humidity levels in the end application.

Nylon 6 moisture absorption

Moisture absorption by nylon 6 has been a source of great study for many years. Although all polymers absorb some amount of moisture, on none does it have such a significant effect as on nylon 6.

Water molecules produce polar bonds with the amide groups in the nylon 6 molecules. Although small, water molecules take up space and displace the nylon 6 molecules. This results in the nylon 6 molecular matrix swelling. Dimensional changes of 0.7% can result in nylon 6 parts from the "as-molded" state to equilibrium at 50% R.H. environments. This change in nylon 6 occurs in approximately 150 days for a 0.0 60 inch (1.5 mm) thick part. (Ref 17) Molecular mobility is increased through the absorption of water. The increase in spacing between nylon 6 molecules lowers the secondary forces allowing easier translational motion.

The absorption of moisture by nylon 6 is a completely reversible physical reaction. Drying in an oven will drive off all but a small percentage of the water molecules which can only be removed through dissolution of the nylon 6 molecular matrix. The rate of absorption/desorption varies with type of nylon 6 as well as temperature and relative humidity of nylon 6. Addition of fillers reduces the effect of moisture both due to volume reduction of the amount of nylon 6 polymer in the mixture, and by sharing the attraction of the molecules somewhat reducing polarity and the available space for moisture molecules. Reinforcements reduce the effects more than fillers due to nylon 6 strong affinity for reinforcement. In addition to the mechanisms which take place with fillers, the adhesion of the nylon 6 molecular matrix to dimensionally stable reinforcements is stronger than than polar bonding of the water molecules and it dominates.

Nylon 6 processing

Another area where moisture has significant effects on nylon 6 is in processing. Heated to moulding temperatures while wet (ie., >0.2 % water) will result is hydrolytic degradation and a significant loss of physical properties. (Hydrolytic degradation is a chemical reaction which occurs at high temperature with some polymers in the presence of water. It causes primary bonds in the molecular chains to be severed thus reducing molecular weight.) Over drying (ie., <0.08% water) will remove the nylon 6 plasticizing effect of the water molecules and make the resin very viscous and hard to flow. The plasticizing effect in processing has to do with mobility and relative spacing of the nylon 6 molecules, the same influence as on physical properties. This low level of moisture does not cause significant degradation during processing. The absorption of moisture by nylon 6 must be considered in mould making. The shrinkage factor of nylon 6 used in designing the mould must take the the potential for change in post moulded dimensional into account. Although moisture causes problems in working with nylon 6, it does contribute to: better dyeability, toughness, softness and greater flexibility in nylon 6 parts. br>
Located in Northamptonshire, England, Ryan Plastics is a nylon 6 engineer and nylon 6 supplier to the UK, internationally, and locally within the East Midlands and East Anglia regions. If you require any more information about nylon 6 contact us on 01604 811395. Our address is 78 High Street, Earls Barton, Northamptonshire, England, UK, NN6 0JG.