Plastic absorbs water, but the amount varies greatly depending on the specific type of plastic. While some plastics absorb barely any moisture, others can take up several percent of their own weight when exposed to humidity or liquid water. Understanding this interaction is important because absorbed water can change a plastic’s physical, mechanical, and electrical characteristics.
The Molecular Mechanism of Water Uptake
Water uptake in plastic involves absorption, which is distinct from adsorption. Absorption is when water molecules move beyond the surface and penetrate the entire internal volume of the material. Adsorption is the attachment of water molecules to the material’s outer surface.
The main driver for water movement into the plastic is diffusion. Water molecules slowly migrate through the “free volume” between polymer chains. This movement continues until the concentration of water inside the plastic reaches an equilibrium with the surrounding humidity or liquid.
The chemical attraction between the water and the plastic determines how much water the material ultimately holds. Water molecules are highly polar. If the plastic contains polar chemical groups, such as amide (-CONH-) or hydroxyl (-OH) groups, these form strong hydrogen bonds with the water molecules, pulling them into the polymer structure. These hydrogen-bonded water molecules are often referred to as “bound water” and are responsible for the most significant changes in material properties.
Factors Determining Absorption Variability
One of the most significant factors determining absorption variability is the polymer’s inherent hydrophilicity, or “water-loving” tendency. Polymers like Nylon (polyamides) contain numerous polar amide groups that readily form hydrogen bonds with water, leading to absorption rates that can reach several percent of the material’s weight.
By contrast, non-polar plastics like Polyethylene (PE) or Polypropylene (PP) are largely composed of carbon and hydrogen atoms, lacking the chemical sites for hydrogen bonding. These materials are hydrophobic and have negligible water absorption, often less than 0.1%. Polytetrafluoroethylene (PTFE), commonly known as Teflon, is notable for having virtually zero water absorption.
Another structural factor is the material’s degree of crystallinity. Plastics are usually semicrystalline. Water molecules cannot penetrate the tightly packed crystalline zones, so absorption is restricted almost entirely to the amorphous sections. Consequently, a plastic with a lower percentage of crystallinity generally absorbs more water than a highly crystalline version of the same polymer.
Environmental conditions, particularly temperature and humidity, also dictate the final absorption level. Higher temperatures increase the molecular motion within the plastic, speeding up diffusion. Relative humidity directly controls the maximum amount of water a plastic will absorb; higher humidity leads to greater equilibrium moisture content.
Practical Effects of Moisture on Plastic Properties
The presence of absorbed moisture has several tangible effects on the performance of a plastic part, making it a major consideration in engineering and manufacturing.
Dimensional Stability
One immediate consequence is a loss of dimensional stability, which is the material’s ability to maintain its original size and shape. As water molecules wedge themselves between the polymer chains, they force the material to physically swell. This swelling can cause warping or failure in precision components designed with tight tolerances.
Mechanical Performance
Moisture also significantly alters the mechanical performance of the material. Absorbed water molecules act as a plasticizer, essentially lubricating the polymer chains and making them easier to move. This internal lubrication causes a reduction in the material’s glass transition temperature, leading to decreased stiffness, lower tensile strength, and reduced impact resistance. For example, Nylon is often tougher when it has absorbed some moisture, but its stiffness and hardness are reduced.
Electrical Properties
Finally, water absorption compromises the electrical properties of plastics, which are often used as insulators. Water has a high dielectric constant compared to most plastics, so its presence within the material increases the overall dielectric constant and the dissipation factor. This means that the plastic becomes a less effective electrical insulator, which is a significant problem for electronic enclosures and components exposed to humid conditions.