Nylon is a family of synthetic polymers known as polyamides, recognized for their strength and versatility in applications ranging from textiles to engineering plastics. Nylon is hygroscopic, meaning it actively attracts and absorbs moisture from the surrounding environment. While it absorbs significantly less moisture than natural fibers like cotton or wool, it does not repel water completely. Understanding this characteristic is important because water absorption fundamentally changes nylon’s physical and mechanical properties.
The Chemical Reason Nylon Interacts With Water
The root cause of nylon’s water absorption lies in the presence of specific chemical groups along its polymer chain. Nylon is classified as a polyamide because its long molecular structure is built from repeating amide groups (\(\text{-CONH-}\)). These amide linkages are highly polar, possessing distinct regions of electrical charge.
The polarity of the amide group allows it to readily form strong hydrogen bonds with water molecules (\(\text{H}_2\text{O}\)). Water molecules are strongly attracted to the oxygen atom in the nylon’s carbonyl group (\(\text{C=O}\)), drawing water into the bulk structure of the material.
Once inside the polymer structure, the water molecules act as a plasticizer. They insert themselves between the long nylon chains and bond with the amide groups, disrupting the strong internal hydrogen bonds that normally hold the polymer chains tightly together. This allows the nylon chains to slide past one another more easily.
This mechanism distinguishes nylon from purely hydrophobic polymers, such as polyethylene, which lack these polar sites. The ability to bond internally means nylon absorbs moisture throughout its entire structure. The degree of absorption varies between different nylon types based on the ratio of water-attracting amide groups to water-repelling carbon segments.
Standard Moisture Regain Rates for Nylon
The standardized measurement used to quantify a fiber’s water absorption is called “moisture regain.” This value is expressed as the percentage of water absorbed by a material relative to its oven-dry weight, measured under standard atmospheric conditions (typically 65% relative humidity and \(70^{\circ}\)F).
Common textile-grade nylons, such as Nylon 6 and Nylon 6,6, display low to moderate moisture regain. Nylon 6,6 typically has a regain rate around \(4.0\) to \(4.5\%\). Nylon 6 can exhibit a slightly higher regain, ranging from \(2.8\) to \(5.0\%\).
These values are low compared to natural fibers. Cotton exhibits a moisture regain between \(7\) and \(11\%\), and wool can reach up to \(18\%\). This confirms that while nylon is hygroscopic, it is a low-absorber relative to materials favored for inherent absorbency.
These regain rates reflect equilibrium with atmospheric humidity, but if submerged, nylon can absorb substantially more water until saturated. For instance, Nylon 6 can absorb up to \(10\%\) of its weight when fully saturated. This difference between standard regain and saturation capacity is important for applications involving constant water contact.
Practical Consequences of Water Absorption
The absorption of water has several measurable effects on the performance and behavior of nylon products. One primary consequence is a change in dimensional stability, relevant for engineered parts and tight-fitting clothing. As water molecules are drawn into the polymer structure, the nylon material swells, leading to an increase in volume.
For materials like Nylon 6,6, full saturation can cause a dimensional change of up to \(3.5\%\), which can affect the fit of precision components. This swelling is reversible, but the material may shrink slightly upon drying, which is a concern for products requiring consistent sizing.
Water absorption also temporarily reduces the material’s mechanical strength, linked to the plasticizing effect of water. The presence of water molecules between the polymer chains decreases the material’s tensile strength and stiffness. Nylon ropes, for example, can experience a \(10\) to \(20\%\) reduction in strength when wet, a factor that must be accounted for in load-bearing applications.
Another element is drying time. Despite being a low absorber, nylon can take a surprisingly long time to dry because the absorbed water is chemically bound to the amide groups. The hydrogen bonds holding the water molecules must be broken for the water to evaporate, meaning large-diameter nylon items can take more than a week to dry completely.
In athletic and outdoor apparel, water absorption affects comfort through moisture handling. Nylon absorbs sweat into the fiber itself, which contrasts with wicking, where moisture is transported along the surface. This means that while nylon pulls some moisture away from the skin, its low surface wetability limits its capacity for rapid moisture transport compared to materials engineered specifically for wicking performance.