Water Absorption of Nylon

What is the water absorption of nylon ?

Polyamide (PA) commonly known as nylon, density of about 1.15g/cm3, is the main chain of the molecule contains repeating amide groups -[NHCO]- thermoplastic resin general term, including aliphatic PA, aliphatic-aromatic PA and aromatic PA. aliphatic PA varieties of which many, large production, widely used, and its name from the synthesis of Its name is determined by the number of carbon atoms in the synthesized monomer.

Aliphatic polyamide contains amine groups and carbonyl groups, easy to form hydrogen bonds with water molecules, so the various materials obtained in the use of water is easily absorbed, resulting in plasticizing effect, resulting in material volume expansion, modulus decline, under stress, the occurrence of significant creep.

Polycaprolactam and polyhexanediamine adipate (nylon 6 and nylon 66) are the most commonly used polyamide materials, and as can be seen from the table below, their water absorption is significantly higher than that of the other materials, and they are capable of absorbing up to 10 percent by mass fraction of water from humid air, and 2 to 4 percent by mass fraction of water in normal humidity environments, leading to a variety of changes in mechanical properties.

The effect of water absorption on the properties of nylon

In the case of nylon 6 and nylon 66, for example, a variety of properties change after both absorb water, and many of the changes in properties are related to the amount of water absorbed.

01

Crystallinity and crystal structure

Crystallographic study of nylon 6/66 found that nylon 6/66 are semi-crystalline materials, molding contain crystalline and amorphous regions. In the crystalline region, the molecular chains are in a planar sawtooth conformation, and hydrogen bonds are formed between the chains through amide bonds. In the amorphous region, the molecular chain conformation is random, most of the amide bonds do not interact with each other to form hydrogen bonds, "free" state, but does not exclude a few areas of the formation of localized hydrogen bonds.

In early studies, the crystallinity of nylon was often estimated by density. The density of nylon 6/66 is greater than that of water, and when water is absorbed, the density of both materials rises, and so does the degree of crystallinity. Nylon 6/66 materials that have been tensile oriented often contain some γ-crystals. It was found that the proportion of γ-crystals in nylon materials decreased after water absorption, while the proportion of more stable α-crystals increased.

02

Mechanical properties and molecular motion

The changes in mechanical properties of nylon after water absorption are obvious. Most notably, hardness, modulus and tensile strength decrease, yield point decreases, and impact strength increases.

The molecular motion of nylon 6/66 is studied by nuclear magnetic resonance, dynamic mechanical relaxation and dielectric loss, etc. The study of the transformation of nylon 6/66 materials before and after the absorption of water found that its glass transition temperature (Tg) is more sensitive to moisture, and after the absorption of water, the Tg decreases dramatically. For example, Tg=94°C for nylon 6 with a water content of 0.35% w/w, and Tg=-6°C for 10.33% w/w; Tg=78°C for dry nylon 66, and Tg=40°C when the water content is 11% w/w. It was also found that the decrease of Tg with the increase of water absorption had a staged process. The initial decrease was rapid; after the water absorption mass fraction exceeded a certain value, the decrease was slow.

The critical value is about 2%~4% as reported in various literatures. Nylon 6/66 also exhibits β and γ transitions at lower temperatures, where the β transition is only observed in wet samples and its intensity increases with increasing water absorption. Some studies also found that the increase in the intensity of the β-transition peak was accompanied by a decrease in the γ-transition peak and showed a Tg-like phase.

All of the above phenomena suggest a plasticization-like effect, however, when the test temperature is further reduced beyond a certain critical temperature, the water has the opposite effect in the nylon 6/66 material, similar to cross-linking hardening. The exact value of this critical temperature varies considerably from report to report, and it has been suggested that this is related to differences in conditions such as the frequency of dynamic mechanical testing and the degree of orientation of the sample.

Nylon hardens when subjected to stresses less than the yield point over a long period of time, an effect known as "stress aging". The rate of stress aging is accelerated by water absorption.

03

Dimensional Changes

Nylon 6/66 expands in volume when water is absorbed. When swelling, the dimensional change of the material is not exactly synchronized with the change in water absorption. Nylon 6 fibers swell faster and then slower as the amount of water absorbed changes, while the opposite is true for nylon 6 film. The swelling is anisotropic for samples that have undergone stretching orientation. The swelling was more pronounced in the direction of tensile orientation.

It is found that the intermolecular hydrogen bonding orientation of nylon 6/66 is closer along the stretching direction under the action of stretching, so it is believed that the water-absorbing swelling of nylon 6/66 is more obvious along the direction of intermolecular hydrogen bonding.

It can be learned from this, nylon water absorption, to a certain extent, affects the dimensional stability and electrical properties, especially thin-walled parts of the thickening of the greater impact; water absorption will also greatly reduce the mechanical strength of the plastic. In the selection of materials, should take into account the use of the environment and other components with the impact of precision.

Now the common practice is to enhance the method through the fiber, can reduce the resin water absorption, so that it can work in high temperature, high humidity. There are also methods of adding phenolic resins such as phenolic resins and polyvinylphenol, as well as the addition of inorganic nanoparticles to reduce the water absorption rate of nylon.

Nylon water absorption in different fields of application

Nylon is a synthetic material used in a wide variety of applications and is known for its superior strength, abrasion resistance and chemical resistance. However, one surprising property, the ability of nylon to absorb water, is often overlooked. In this article, we will take an in-depth look at the nature of nylon's water absorption, its mechanism, and its applications in different fields.

Water Absorption Nature of Nylon:

Nylon is a polyamide-based synthetic material with considerable water absorption. The amide groups in the molecular chain of nylon are hydrophilic and thus absorb and retain water molecules. The water-absorbent nature of nylon makes it unique in a variety of applications.

Mechanism of water absorption in nylon:

The mechanism by which nylon absorbs water can be divided into two steps:

Adsorption of water molecules: Hydrogen bonds are formed between the amide groups on the surface of the nylon and the water molecules, enabling the water molecules to attach to the surface of the nylon.

Infiltration of water molecules: water molecules enter the interior of the nylon material through infiltration, penetrating along gaps or defects in the molecular chain and causing the material to swell.

These two steps work together to enable the nylon to absorb large amounts of water.

Application areas where nylon absorbs water:

Textile Industry: Nylon fibers have a wide range of applications in making textiles with good moisture regulation properties. These textiles absorb body sweat in hot weather, making the wearer more comfortable.

Medical: Nylon's water-absorbent properties are also important in medical supplies, such as dressings and moisturizing therapies. It helps wounds heal, maintains a moist environment and promotes tissue regeneration.

Cosmetics: Nylon wicking is used in some cosmetic products to adjust the texture and consistency, making it easier to apply and absorb.

Food Packaging: Some food packaging materials include nylon because it absorbs water from food products, maintaining their freshness and flavor.

Industrial applications: Nylon water absorption is also valuable in certain industrial applications such as filter materials, oil-water separation, and wet grinding.

Control and Improvement of Nylon Water Absorption:

The water-absorbent nature of nylon can be adjusted by controlling the composition, structure and handling of the material. For example, the water-absorbent properties of nylon can be improved or reduced by changing the degree of polymerization of the nylon, by adding special hydrophilic components, or by employing post-processing methods. This control allows nylon to be customized to the needs of specific applications.

The unique nature of nylon's water absorption provides interesting opportunities for applications in a variety of fields. In the future, with advances in materials science, we can expect more innovations, including the development of higher performance and more controllable nylon water absorption materials. This will help to meet the different needs for water absorption performance in different fields and promote the expansion of nylon applications in more fields.

Nylon is a versatile synthetic material, and its water-absorbent properties make it useful in a variety of fields, including textiles, medical, food packaging and industrial applications. By controlling and improving the water absorption properties of nylon, we can better meet the needs of a variety of applications and look forward to seeing more innovations in nylon's water absorption properties in the future. The uniqueness of this property allows nylon to play an important role in expanding its applications.