Ultraviolet (UV) light is a form of electromagnetic radiation, residing just beyond the violet end of the visible light spectrum. It carries more energy than visible light, with wavelengths typically ranging from 10 to 400 nanometers. The UV spectrum is broadly categorized into three types: UVA, UVB, and UVC. Understanding how this light interacts with various materials, particularly plastics, helps explain many everyday phenomena.
How UV Light Interacts with Materials
When UV light encounters a material, it can interact in several ways: absorption, reflection, or transmission. Absorption occurs when the material takes in light energy, converting it into heat or causing changes to its electronic structure. This energy transfer can lead to chemical changes. Reflection happens when UV light bounces off the surface. Transmission describes the process where UV light passes directly through a material. Whether UV light is absorbed, reflected, or transmitted depends on the material’s molecular composition.
Which Plastics Block or Allow UV Light
Plastics exhibit varied responses to UV light, determined by their chemical structure.
Plastics with Lower Inherent UV Resistance
Polyethylene Terephthalate (PET), commonly used for beverage bottles, offers poor UV protection unless treated. While PET absorbs UV light up to 320 nanometers, it often allows UVA to pass through, affecting packaged contents.
Polyvinyl Chloride (PVC) is susceptible to UV degradation without protective additives, leading to yellowing and loss of mechanical strength. Stabilizers can significantly improve its UV resistance.
High-Density Polyethylene (HDPE) and Low-Density Polyethylene (LDPE), found in milk jugs and plastic bags, are not inherently UV resistant. While they transmit some UV, additives can enhance their protective qualities.
Polypropylene (PP), used in many containers, typically has poor UV resistance without additives, and prolonged exposure can cause it to become brittle.
Plastics with Higher or Engineered UV Resistance
Acrylic (PMMA) is clear and allows significant UV light to pass, similar to glass. However, it is inherently stable to UV light and can be engineered to block up to 98% of harmful UV radiation below 400 nanometers.
Polycarbonate (PC) is strong and naturally absorbs most UVB and UVC radiation. Untreated polycarbonate allows UVA to pass through and can yellow or degrade over time due to UV absorption.
What Affects UV Light Transmission Through Plastic
Several factors influence how much UV light penetrates plastic.
Material Thickness
Material thickness plays a role, with thicker plastics generally offering greater UV blocking capabilities. This increased material volume provides more opportunities for UV photons to be absorbed or scattered.
Additives
The incorporation of additives significantly alters UV transmission. UV stabilizers and absorbers can be mixed into the plastic during manufacturing. UV absorbers convert harmful UV energy into less damaging forms, such as heat, or by re-emitting it at safer wavelengths. Certain stabilizers react with free radicals generated by UV exposure, mitigating degradation.
Color and Opacity
Color and opacity also impact UV transmission. Darker or opaque plastics, especially those containing carbon black, block more UV radiation than clear plastics of the same material. Pigments such as titanium dioxide also contribute to UV protection by scattering and absorbing UV light. Even clear plastics can be made UV-resistant through the strategic use of these additives.
Real-World Effects of UV Passing Through Plastic
The transmission or blocking of UV light through plastic has several practical implications.
Protection and Degradation
For sun protection, plastics in eyewear or car windows require specific UV-blocking properties to shield users from harmful radiation. Food, beverages, and medications in plastic containers can degrade if packaging does not adequately block UV light.
UV exposure also causes material degradation in plastics over time. This process, known as photodegradation, can lead to the material becoming brittle, discolored, and losing strength. Common signs include a chalky appearance, color changes, and cracking. Many outdoor plastic products are designed with UV resistance to prolong their lifespan.
Desired UV Transmission
In some applications, UV transmission through plastic is intentionally desired. For instance, in UV curing of resins or certain sterilization processes, specific plastics are chosen for their ability to allow UV light to pass through. This controlled transmission is integral to the effectiveness of these specialized applications.