Polylactic Acid (PLA) is one of the most popular materials in 3D printing due to its ease of use. Post-processing often involves using a solvent to smooth the surface, and acetone is the standard chemical employed for this task with other plastics, such as ABS. The effectiveness of acetone as a solvent for PLA is a common question among users. This article explores the specific chemical interactions between the two substances to determine if this common household solvent can dissolve the printing plastic.
The Specific Interaction of Acetone and PLA
Standard, room-temperature acetone generally does not dissolve or aggressively smooth Polylactic Acid (PLA) as it does other common 3D printing polymers. Unlike Acrylonitrile Butadiene Styrene (ABS), which readily dissolves in acetone vapor, PLA’s chemical structure provides significant resistance to this solvent. Applying liquid acetone or exposing the plastic to its vapor will not result in the rapid, uniform surface melting needed for effective layer line smoothing. Therefore, acetone is an impractical choice for post-processing PLA prints.
When PLA is exposed to acetone, the results are often inconsistent and undesirable for surface finishing. Instead of dissolution, the plastic may exhibit minor surface softening, resulting in a tacky or sticky texture that ruins the print’s aesthetic quality. Prolonged exposure can sometimes lead to stress-cracking or whitening, where the material structurally degrades rather than dissolving smoothly. This limited interaction is due to a fundamental chemical incompatibility that prevents the solvent from effectively breaking down the polymer chains.
Why Polylactic Acid Resists Acetone
The lack of dissolution is rooted in the molecular composition of both the solvent and the polymer. PLA is a thermoplastic polyester whose long molecular chains are relatively stable against acetone’s chemical action. This stability is enhanced by the semi-crystalline nature of most PLA filaments. The highly ordered crystalline regions within the material are particularly resistant to solvent penetration, making it difficult for acetone molecules to break apart the polymer structure.
The principle of “like dissolves like” helps explain this resistance by comparing the two substances’ polarity profiles. This relationship is quantified by the Hansen Solubility Parameters (HSP), which measure a substance’s dispersive, polar, and hydrogen-bonding characteristics. Acetone’s parameters do not closely match those of the PLA polymer, indicating a weak attractive force between the two molecules. Due to this poor chemical compatibility, the solvent lacks the necessary strength to overcome the internal forces holding the PLA chains together. While acetone may dissolve small, unreacted lactide monomers, it cannot attack the larger polymer structure.
Alternative Solvents and Smoothing Methods for PLA
Since acetone proves ineffective for reliable smoothing, users must turn to specialized chemicals or mechanical techniques. Ethyl Acetate (ETAc) is often cited as a much more effective solvent for PLA than acetone. This solvent, found in some non-acetone nail polish removers, has better chemical compatibility with PLA’s polyester structure, allowing it to dissolve and smooth the surface reliably. Dichloromethane (DCM), also known as Methylene Chloride, is another solvent highly effective at dissolving PLA.
These specialized solvents require significant safety precautions due to their industrial nature. Ethyl acetate is highly flammable, and proper ventilation is required when working with it. Dichloromethane is a restricted solvent with serious health hazards, and its use should be limited to controlled environments with strict Personal Protective Equipment (PPE) protocols. Using any strong chemical solvent requires careful handling, adequate ventilation, and protective gear, such as gloves and respirators.
For those seeking safer, non-chemical alternatives, mechanical and physical smoothing methods are widely available. Traditional sanding and polishing can be used to manually remove layer lines, starting with coarse grits and progressing to finer ones for a smooth finish. Alternatively, applying a two-part epoxy coating, such as a brush-on resin, can fill in the microscopic layer lines and provide a smooth, glossy surface. These non-solvent methods are often more time-consuming but avoid the risks associated with handling volatile and potentially toxic chemicals.