Polylactic acid (PLA) is a thermoplastic derived from renewable resources like corn starch or sugarcane, making it a popular, bio-based alternative to petroleum-derived plastics. Its ease of use, low melting temperature, and minimal warping have established it as the most used material in desktop fused deposition modeling (FDM) 3D printing. While PLA is used for prototyping and packaging, its durability is often questioned for outdoor applications. The central concern revolves around its resistance to ultraviolet (UV) radiation, a significant factor for any plastic used in direct sunlight.
How UV Radiation Affects PLA
PLA is susceptible to degradation when exposed to UV light over extended periods, making it an inferior choice for long-term outdoor use. The high-energy photons in sunlight initiate chemical changes that physically weaken the material over time. This degradation process is significantly accelerated when PLA is exposed continuously to the elements.
The most noticeable physical consequence is a loss of mechanical strength and structural integrity. As the polymer breaks down, the material becomes weaker, which can lead to cracking, warping, or complete structural failure under load. This loss of strength is linked to an increase in brittleness, causing rigid parts to snap or fracture easily rather than bending.
UV exposure also causes aesthetic deterioration. Discoloration is common, often presenting as yellowing or clouding of the surface. Furthermore, the pigments and dyes used to color the PLA filament are not always UV-stable, resulting in color fading. High temperatures and humidity alongside intense UV radiation accelerate this process.
The Chemical Process of Photodegradation
The degradation of PLA in sunlight is a complex chemical process known as photodegradation, driven by the energy from UV radiation. UV light is powerful enough to break the chemical bonds that form the long polymer chains of the plastic. This energy transfer is often facilitated by impurities present in the polymer or by the presence of oxygen and moisture.
The primary destructive mechanism is photo-oxidation, where UV energy interacts with the polymer structure and atmospheric oxygen. This interaction leads to the formation of highly reactive molecules called free radicals, which then initiate a chain reaction within the plastic.
The critical result of this chemical attack is chain scission, the breaking of the main polymer backbone. This bond cleavage occurs primarily at the ester bonds in the PLA structure, chopping the long, high-molecular-weight polymer chains into many shorter segments. The molecular weight reduction directly accounts for the observed physical effects, leading to the rapid loss of strength and increased brittleness.
Practical Methods for UV Protection
For users who need to employ PLA parts in outdoor conditions, several practical mitigation strategies can significantly extend the material’s lifespan.
Protective Coatings
The most effective and accessible method involves applying a protective coating to the finished part. UV-resistant paints, such as those formulated with acrylics or enamels for exterior use, create an opaque barrier that absorbs or reflects the damaging UV radiation before it can reach the underlying PLA.
A clear UV-blocking topcoat or sealant can also be used if the original color or surface finish needs to be preserved. These coatings work by incorporating specialized chemical stabilizers that intercept the UV photons. For the best adhesion and long-term protection, the PLA object should be thoroughly cleaned and often primed before the protective layer is applied.
Specialized Filaments
Another strategy is to utilize specialized PLA formulations that incorporate UV stabilizers during the manufacturing process. Additives, such as carbon black or inorganic nanoparticles, are mixed into the filament to enhance its resistance to light-induced degradation. Carbon black is highly effective at absorbing UV light and is a common additive for plastics intended for outdoor use.
Environmental Management
Careful environmental placement and management is also effective. If a PLA object does not need to be under continuous load or in direct, intense sunlight, relocating it to a shaded area or bringing it indoors when not actively in use can dramatically slow the degradation process. Reducing the exposure time will lower the total accumulated UV dose and preserve the material’s integrity and aesthetic quality for much longer.