Polylactic Acid (PLA) is a widely used 3D printing material due to its ease of use and derivation from renewable resources like corn starch. As a polymer, its structure suggests a degree of water compatibility, but its functional performance when wet is complicated by the nature of the printing process. Understanding how PLA interacts with moisture is important for any functional application requiring prolonged exposure to liquids. The critical distinction lies in whether the material can simply repel a splash or if it can maintain structural integrity and prevent permeation over an extended period.
Understanding Water Resistance vs. Waterproofing
PLA is inherently water-resistant, meaning the raw polymer itself does not readily dissolve or suffer immediate damage from brief contact with water. The chemical structure of the plastic is not hydrophilic, allowing it to withstand simple exposure such as accidental splashes, rain, or short-term submersion. However, a 3D-printed PLA object is not truly waterproof because of the manufacturing method used in Fused Deposition Modeling (FDM).
The layer-by-layer nature of FDM printing creates microscopic gaps and voids between the deposited lines of filament. These imperfections act as capillaries, allowing water to slowly seep into the object’s internal structure over time, especially under continuous hydrostatic pressure. While the solid plastic material resists the water, the architectural structure of the printed part does not offer a complete seal. This porosity makes the part unsuitable for applications requiring it to hold water for long periods or survive constant outdoor exposure.
Water ingress is significantly affected by the print settings used. Parts with low infill density or thin walls will absorb water faster. High-quality prints with increased wall thickness and optimized layer adhesion exhibit greater resistance, though they still cannot be considered fully waterproof. The temperature of the liquid also plays a role, as PLA’s resistance is notably lower when exposed to hot water, which accelerates the softening and penetration process.
Material Degradation from Water Exposure
Prolonged exposure to water, or even high humidity, begins two distinct processes that compromise the integrity of the PLA material. The first is hygroscopicity, which describes PLA’s tendency to absorb moisture from the surrounding air or environment. This moisture absorption is a major concern for the filament itself, causing swelling and poor print quality, but it also affects the printed part by slightly increasing its volume and mass.
The second, and more significant, process is hydrolysis, which is the chemical breakdown of the polymer chains by water molecules. PLA is a polyester, and the presence of water causes the ester bonds in the long molecular chains to cleave, a reaction known as chain scission. This process fragments the polymer into shorter segments, leading to a substantial reduction in the material’s average molecular weight.
The hydrolytic degradation results in a loss of mechanical properties, manifesting as increased brittleness and a significant decrease in tensile strength. This reaction is notably accelerated by elevated temperatures, which is why hot water or warm, humid conditions cause failure much faster than cold water. Over a long enough timeline, this breakdown is the mechanism by which PLA is considered biodegradable, as the fragmented chains eventually become small enough to be consumed by microorganisms.
Post-Processing Methods to Enhance Resistance
For applications where a PLA print must function reliably in a wet environment, post-processing methods are necessary to create a complete moisture barrier. The initial step is to optimize the print settings by increasing the number of perimeter shells to at least three, which minimizes internal voids and maximizes outer wall density. Even with maximized print settings, a coating is required to seal the microscopic layer lines.
Applying a sealant is the most practical way to achieve true waterproofing for a PLA print. Epoxy resins are highly effective, as they cure into a hard, non-porous layer that completely fills the surface imperfections and micro-gaps. This method creates a robust, continuous shell that prevents water from contacting the underlying PLA, stopping both permeation and hydrolysis.
Alternative protective coatings include acrylic or polyurethane paints and varnishes. These sealants are simpler to apply and can be effective for resistance against splashes or brief exposure, but they may not offer the same long-term protection as a thick, two-part epoxy resin. Regardless of the chosen coating, the goal is the same: to wrap the porous 3D-printed structure in an unbroken, water-impermeable shell that transforms the part from merely water-resistant to functionally waterproof.