Why does nylon have strong water absorption, and what effect does it have on performance after water absorption?
2023-02-02
Polyamide (PA), commonly known as Nylon, has a density of about 1.15g/cm 3 and is a general term for thermoplastic resins containing repeated amide groups-[NHCO]-in the main chain of the molecule, including aliphatic PA, aliphatic-aromatic PA and aromatic PA. . Among them, aliphatic PA has many varieties, large output and wide application, and its naming depends on the specific number of carbon atoms in the synthetic monomer. Because aliphatic polyamide contains amine groups and carbonyl groups, it is easy to form hydrogen bonds with water molecules, so the various materials obtained are easy to absorb water when used, resulting in a plasticizing effect, resulting in volume expansion and a decrease in modulus of the material. Significant creep occurs. Polycaprolactam and polyhexamethylene adipate (Nylon 6 and Nylon 66) are the most commonly used polyamide materials, as can be seen from the table below, their water absorption is significantly higher than other materials, and their highest absorption from humid air Moisture with a mass fraction of 10% can also absorb moisture with a mass fraction of 2% to 4% in a general humidity environment, resulting in changes in various mechanical properties. Effect of Nylon Absorbing Water on Properties Taking nylon 6 and nylon 66 as examples, after absorbing water, many properties change, and the changes of many properties are related to the amount of water absorbed. 01 Crystallinity and Crystal Structure The crystallographic study of nylon 6/66 found that nylon 6/66 is a semi-crystalline material, which contains both crystalline and amorphous regions after molding. In the crystalline region, the molecular chains are in a planar zigzag conformation, and hydrogen bonds are formed between chains through amide bonds. In the amorphous region, the molecular chain conformation is random, most of the amide bonds do not interact to form hydrogen bonds, and are in a "free" state, but it is not ruled out that a few regions form local hydrogen bonds. In earlier studies, nylon crystallinity was often estimated by density. The density of nylon 6/66 is higher than that of water. After absorbing water, the density of these two materials increases instead, and the crystallinity also increases. Nylon 6/66 materials that have been stretch oriented often contain some γ-crystals. The study found that after water absorption, the proportion of γ-crystals in nylon materials decreased, while the proportion of more stable α-crystals increased. 02 Mechanical Properties and Molecular Motion The change in mechanical properties of nylon after water absorption is obvious. The most important is the decrease of hardness, modulus and tensile strength, the decrease of yield point and the increase of impact strength. The molecular motion research of nylon 6/66 includes methods such as nuclear magnetic resonance, dynamic mechanical relaxation and dielectric loss. The research on the transformation of nylon 6/66 before and after water absorption shows that its glass transition temperature (Tg) is relatively sensitive to moisture. After water absorption, Tg dropped significantly. For example, Tg=94℃ when the water content of nylon 6 is 0.35%w/w, Tg=-6℃ when the water content is 10.33%w/w; Tg=78℃ when the water content is 11%w/w of dry nylon 66 =40°C. At the same time, it is found that the process of Tg decrease with the increase of water absorption has a stage. The initial decline is rapid; when the water absorption mass fraction exceeds a certain value, the decline is slow. Based on various literature reports, the critical value is about 2% to 4%. Nylon 6/66 also exhibits beta and gamma transitions at lower temperatures, where the beta transition is only observed in wet samples, and its strength increases with water absorption. Some studies have also found that the increase in the intensity of the β-transition peak is accompanied by the decrease of the γ-transition peak, and presents a phase similar to Tg. The above phenomena all indicate the effect of plasticization. However, when the test temperature is further reduced and exceeds a certain critical temperature, the effect of moisture in the nylon 6/66 material is reversed, similar to cross-linking hardening. The specific value of this critical temperature varies greatly in different reports, and some people suggest that this is related to the difference in the frequency of dynamic mechanical testing, the degree of orientation of the sample, and other conditions. Nylon will harden after being subjected to stress less than the yield point for a long time. This effect is called "stress aging". After water absorption, the rate of stress aging is accelerated. 03 size change Nylon 6/66 will expand in volume after absorbing water. When expanding, the dimensional change of the material and the change of water absorption are not completely synchronized. Nylon 6 fiber expands quickly and then slowly with the change of water absorption; while nylon 6 film is the opposite. After stretching the oriented sample, the expansion is anisotropic. Swelling is more pronounced in the direction of stretch orientation. Studies have found that under the action of stretching, the intermolecular hydrogen bond orientation of nylon 6/66 is closer to the stretching direction, so it is believed that the water absorption expansion of nylon 6/66 is more obvious along the direction of the intermolecular hydrogen bond. It can be known from this that nylon has high water absorption, which affects dimensional stability and electrical properties to a certain extent, especially the thickening of thin-walled parts; water absorption will also greatly reduce the mechanical strength of plastics. When selecting materials, the influence of the use environment and the accuracy of cooperation with other components should be taken into account. Now the common practice is to use fiber reinforcement to reduce the water absorption of the resin so that it can work under high temperature and high humidity. There are also methods of adding phenolic resins such as phenolic resins and polyvinylphenols, and adding inorganic nanoparticles to reduce the water absorption of nylon.
