Polyethylene Terephthalate Glycol (PETG) is a widely used thermoplastic polymer known for its durability, clarity, and ease of processing. A modified version of Polyethylene Terephthalate (PET), PETG is popular in manufacturing and hobbyist applications. Understanding how this material interacts with water is necessary for assessing its performance. PETG’s resistance to liquid water and its tendency to absorb atmospheric moisture are two distinct properties that determine its suitability for various environments.
PETG’s Resistance to Liquid Water
PETG is a polyester characterized by its hydrophobic nature, meaning it effectively repels bulk liquid water. Its chemical structure provides resistance to chemical degradation when submerged or exposed to standing water. The polymer chains do not readily react with water molecules, ensuring the material’s physical form and strength remain unaffected by direct contact with liquids.
The material performs excellently in liquid environments. PETG retains high tensile strength and impact resistance even after prolonged immersion, making it a reliable choice for long-term liquid exposure applications, such as water bottles, food storage containers, and outdoor components.
The water absorption rate of a finished PETG part is very low, often measured at less than 0.2% by weight after a 24-hour immersion. This minimal absorption confirms its suitability for applications where the material must resist physical breakdown.
Understanding PETG’s Hygroscopicity
Despite being highly resistant to liquid water, PETG is a significantly hygroscopic material, meaning it absorbs moisture from the surrounding air as water vapor. This distinction is important, as the material’s performance relies on managing the moisture content trapped within its molecular structure. The rate of absorption varies based on ambient humidity and temperature.
Absorbed moisture severely compromises the material during manufacturing processes like extrusion or 3D printing. When wet PETG is heated rapidly, the trapped water turns into steam, which expands violently inside the material. This rapid vaporization leads to defects such as bubbling, popping sounds, and inconsistent extrusion.
The presence of moisture during processing also weakens the final product by reducing the bonding strength between layers, leading to poor layer adhesion and a rough surface finish. Absorbed moisture can also cause dimensional instability. Therefore, moisture must be removed from the raw material before it is processed.
Maintaining Stability in High-Moisture Environments
To ensure optimal performance when using raw PETG material, such as filament or pellets, it is necessary to counteract its hygroscopic nature through proper storage and pre-processing. The most effective method is a drying process, often called baking, which involves heating the material to a precise temperature, typically between 55 and 65 degrees Celsius, for several hours.
This controlled heat drives the absorbed water vapor out of the polymer without melting the material. Drying times commonly range from four to twelve hours, depending on saturation. A dedicated filament dryer, food dehydrator, or temperature-controlled oven is necessary to maintain the consistent heat required for this process.
Once dried, PETG must be stored in an airtight environment to prevent the rapid re-absorption of atmospheric moisture. Storage containers with rubber seals and a desiccant like silica gel are highly effective. For finished parts used in damp environments, applying an external protective coating can enhance the material’s liquid water resistance.