Polyamide fiber is commonly known as nylon (Nylon), English name Polyamide (PA), density 1.15g/cm3, is general term for thermoplastic resins containing repeating amide groups—[NHCO]— on molecular backbone, including aliphatic PA, aliphatic-aromatic PA and aromatic PA. Among them, aliphatic PA has many varieties, large output and wide application, its name is determined by specific carbon number of synthetic monomer.
Since aliphatic polyamide contains amine and carbonyl groups, it is easy to form hydrogen bonds with water molecules. Therefore, various materials obtained are easy to absorb water during use and produce a plasticizing effect, which leads to volume expansion of material, decrease of modulus, and obvious creep under action of stress.
Polycaprolactam and polyhexamethylene adipate (nylon 6 and nylon 66) are the most commonly used polyamide materials. They can absorb up to 10% mass fraction of water from humid air, and can also absorb 2% to 4% by mass in a general humidity environment, resulting in a variety of changes in mechanical properties.

 

(Water absorption of PA6 and PA66 is significantly higher than other materials)

Take nylon 6 and nylon 66 as examples. After nylon 6/66 absorbs water, many properties change, changes of many properties are related to water absorption.

Crystallographic study of nylon 6/66 found that nylon 6/66 is a semi-crystalline material, which contains crystal regions and amorphous regions after molding. In crystal region, molecular chains are in a plane zigzag conformation, hydrogen bonds are formed between chains through amide bonds. In amorphous region, molecular chain conformation is random. Most of amide bonds do not interact to form hydrogen bonds and are in a "free" state. However, it is not excluded that a few regions have formed local hydrogen bonds.
In early studies, nylon crystallinity was often estimated by density. Density of nylon 6/66 is higher than that of water. After absorbing water, density of these two materials increases, crystallinity also increases. Stretch-oriented nylon 6/66 material often contains some γ-crystals. Studies have found that ratio of γ-crystals of nylon materials decreases after water absorption, while ratio of more stable α-crystals increases.

Change in mechanical properties of nylon after absorbing water is obvious. The most important is decrease in hardness, modulus and tensile strength, decrease in yield point, and increase in impact strength.
Molecular motion studies of nylon 6/66 include nuclear magnetic resonance, dynamic mechanical relaxation, and dielectric loss. It is found that glass transition temperature (Tg) of nylon 6/66 is more sensitive to moisture after absorbing water. Tg dropped drastically. For example, when water content of nylon 6 is 0.35%w/w, Tg=94℃, when 10.33%w/w, Tg=-6℃; dry nylon 66Tg=78℃, when water content is 11%w/w, Tg=40 ℃. At the same time, it is found that process of Tg decreasing with increase of water absorption has a phased nature. Initial decline is rapid; when water absorption mass fraction exceeds a certain value, decline is slow.
Based on various literature reports, critical value is about 2% to 4%. Nylon 6/66 also exhibits β and γ transitions at lower temperatures, where β transitions are only observed in wet samples, and its strength increases with increasing water absorption. Some studies have also found that increase in intensity of β transition peak is accompanied by decrease of γ transition peak, and presents a phase similar to Tg.
Above phenomena all show effect similar to plasticization. However, when test temperature is further reduced and exceeds a certain critical temperature, effect of moisture in nylon 6/66 material is reversed, similar to cross-linking and hardening. Specific value of this critical temperature varies greatly in different reports. Some people have suggested that this is related to difference in conditions such as frequency of dynamic mechanical testing and degree of orientation of sample.
Nylon will harden after being subjected to stress less than yield point for a long time. This effect is called "stress aging". After absorbing water, rate of stress aging increases.

Volume of nylon 6/66 will expand after absorbing water. When swelling, change in size of material and change in water absorption are not completely synchronized. Nylon 6 fiber expands quickly and then slowly with change of water absorption; while nylon 6 film is opposite. After stretching orienting sample, expansion is anisotropic. Expansion is more obvious in direction of stretch orientation.
Studies have found that nylon 6/66 intermolecular hydrogen bonds are oriented in direction of stretching under stretching. Therefore, it is believed that nylon 6/66 swells in direction of intermolecular hydrogen bonds.
From this we can know that nylon has high water absorption and saturated water can reach more than 3%. To a certain extent, dimensional stability and electrical performance are affected, especially thickening of thin-walled parts has a greater impact; water absorption will also greatly reduce mechanical strength of plastic. When selecting materials, consideration should be given to influence of use environment and accuracy of coordination with other components.
Current common practice is to reduce water absorption rate of resin through method of fiber reinforcement, so that it can work under high temperature and high humidity.
There are also methods of adding phenolic resins such as phenolic resin and polyvinylphenol, adding inorganic nanoparticles to reduce water absorption of nylon.