PETG (Polyethylene Terephthalate Glycol) is a popular thermoplastic used in commercial packaging and 3D-printed parts. It is valued for its balance of strength, impact resistance, and ease of use compared to other engineering plastics. Considering its durability, many users consider PETG for outdoor projects, raising the question of how it holds up when exposed to ultraviolet (UV) light from the sun. The answer requires a nuanced assessment of its performance against the intense energy that drives polymer degradation.
How UV Light Degrades Plastics
Sunlight contains high-energy UV photons that initiate photodegradation when absorbed by plastics. This energy excites the polymer chains, causing the breaking of chemical bonds, known as chain scission, which generates highly reactive free radicals.
These free radicals react with oxygen, resulting in photo-oxidation. This process systematically reduces the plastic’s molecular weight, fundamentally altering its composition. Initial signs of this degradation include discoloration, often manifesting as yellowing or chalking on the surface. Over time, the internal molecular damage leads to a loss of mechanical properties, causing the material to become brittle and fragment.
PETG’s Performance Under UV Exposure
PETG is a modified version of Polyethylene Terephthalate (PET). While the added glycol improves its printability and impact strength, it does not provide complete protection against prolonged UV exposure. When subjected to direct, sustained sunlight, PETG’s UV resistance is fair at best, making it unsuitable for long-term outdoor use without a protective coating. Degradation is marked by two primary effects: a rapid change in appearance and a decline in physical strength.
The most immediate sign of sun exposure is yellowing, especially prominent in clear or light-colored filaments. This discoloration results directly from photo-oxidation occurring on the material’s surface. Internal degradation also leads to a measurable loss of mechanical strength and ductility. Accelerated aging tests show that prolonged UV exposure causes a significant reduction in tensile strength and an increase in brittleness.
This loss of toughness means a part that initially had slight flex and resilience will eventually become stiff and prone to cracking under minor stress. While PETG is not instantly destroyed by sunlight, its reliable service life is significantly limited outdoors. Parts exposed to the elements may begin to show signs of failure within weeks to months, depending on the sun’s intensity.
Comparing PETG to Other Common Polymers
When evaluating PETG for outdoor use, its UV resistance is best understood by comparing it to other common 3D printing and engineering plastics. PETG offers an advantage over Polylactic Acid (PLA), which is highly susceptible to both UV light and heat. PLA degrades rapidly outdoors, often losing structural integrity quickly due to its low glass transition temperature.
However, PETG falls short compared to plastics designed for environmental stability, such as Acrylonitrile Butadiene Styrene (ABS) and Acrylonitrile Styrene Acrylate (ASA). While PETG offers a balance of strength and ease of printing, standard ABS generally exhibits better UV performance, though it can still suffer from chalking and discoloration.
ASA is the benchmark for high UV resistance in this group and is superior to PETG. Developed specifically to overcome the poor UV stability of ABS, ASA is the material of choice for demanding outdoor applications like automotive exterior parts. For users needing a part that will maintain its mechanical properties and appearance for many years of continuous sun exposure, ASA is a more appropriate material selection than PETG.
Methods to Improve PETG’s UV Resistance
For applications where PETG must be used outdoors, several mitigation strategies can significantly extend its lifespan and maintain structural integrity. The most effective method is to physically block UV radiation from reaching the polymer surface. Applying an opaque coating, such as paint, prevents UV light transmission and halts the photo-oxidation process.
Using Protective Coatings
For the best results, users should first apply a primer designed for plastics, followed by a topcoat. Ideally, this paint contains zinc oxide, a known UV absorber. If a clear finish is desired, apply a clear coat or varnish formulated with UV blockers. Automotive-grade clear coats are often recommended as they contain durable UV stabilizers designed for long-term sun exposure.
Choosing Stabilized Filaments
The choice of filament color is also a factor; darker colors, particularly black, naturally absorb more radiation and prevent light penetration deeper into the part. Furthermore, some manufacturers offer PETG filaments compounded with internal UV stabilizers. These chemical additives interfere with the free radical formation process, slowing the rate of degradation.