Chris Rockett
production and applications engineer, LightSources, Inc.
As ultraviolet (UV) radiation consists of photons with high energy relative to visible light, it can cause degradation in the form of physical and chemical changes in susceptible materials. The degradation effects of UV are of concern to designers and users of a wide variety of materials that are intended for use and storage outdoors and thereby exposed to sunlight. Accordingly, the published data on material degradation by UV is almost exclusively relevant to that present in sunlight at the earth’s surface, as this represents such a large economic impact.
However, UV-C, the subclass of UV radiation with wavelengths between 200 and 280 nm, is not present in terrestrial sunlight because wavelengths lower than ~300 nm are absorbed by the ozone layer in the upper atmosphere. Therefore, published data on materials’ degradation by UV-C is minimal. As a crudely demonstrative example, a search in Google Scholar for “UV degradation” results in over three million hits, while “UV-C degradation” results in only around 19,000 hits. Granted, sunlight-induced damage may be a reasonable predictor for what could be expected to occur with UV-C exposure, but the high-energy UV-C photons can have unique effects that cannot always be predicted by solar UV exposure.
Knowledge of the effects of UV-C exposure on various materials is useful to manufacturers and users of UV-C disinfection and photochemical equipment. The intent of this article is to give an overview of the broad classes of materials and an explanation of their susceptibility (or lack thereof) to UV-induced degradation. Such an overview must start with a basic lesson in materials science, explaining the three broad classes of materials as classified by their atomic bonding characteristics.
Metals are characterized by metallic bonding, which is defined by tightly packed atoms arranged in a periodic lattice structure all sharing a “cloud” of delocalized electrons. Because of their highly mobile electrons, metals are good conductors of electricity and heat, and readily interfere with electromagnetic radiation such as light and radio waves. This explains why metals are never transparent and almost always reflect light to some degree. Metals are almost entirely unaffected by UV because of the availability of free electrons to absorb photon energy without undergoing energy transitions or bond dissociation. There is some published evidence1 of increases in the corrosion rates of metals immersed in water exposed to UV, but the results are of questionable significance, and the metals that seem to be most susceptible are not good candidates for immersion anyway. These effects are hypothesized to be related to photoelectric effects between the surface oxide layer and the underlying metal. Suffice it to say that for nearly all applications, metals can be considered impervious to UV degradation.
Ceramics are characterized primarily by ionic bonding, the attraction of positively and negatively charged ions arranged in a periodic lattice structure. Most ceramics are metal oxides, though some ceramics are nitrides, borides and carbides that exhibit strong covalent bonding. In contrast to metals, ceramic ions have tightly bound electrons, hence they have a high bond strength, withstand extreme temperatures, are usually extremely chemically inert and are strong electrical insulators. It is this high bond strength and chemical inertness that make ceramics completely unaffected by UV exposure (see Endnote).
Polymers comprise a wide variety of materials that are characterized by the entanglement and interconnection of long molecules (a.k.a. polymer “chains”), which themselves exhibit covalent bonding, typically between organic (i.e., carbon-containing) constituents. Covalent bonding is the sharing of electrons between two or more atoms in order to satisfy the constituent atoms’ propensity to fill their outermost electron orbitals (i.e., a satisfied valence shell). Covalent sharing of electrons is localized (i.e., electron mobility is limited to the nearest bonding atoms), in contrast with metallic bonding, so polymers are almost always electrical insulators and poor conductors of heat. Covalent bonds between organic constituents are also relatively weak compared to metallic and ionic bonds. Therefore, most polymers are susceptible to degradation by UV-C exposure. The high-energy photons have enough energy to promote electrons to higher energy levels and, thereby, dissociate or enable oxidation of covalent bonds. In general, polymers with carbon-carbon double bonds are more susceptible to UV-induced chemical changes.
The reality is that no material exhibits any single, pure type of bonding – they all share some characteristics of other types of bonding. One material exhibiting a combination of ionic and covalent bonding that is of great importance to the UV industry is amorphous silica (SiO2), known by many names such as quartz glass, fused quartz, fused silica or simply by the common misnomer quartz. Fused silica exhibits a random arrangement of silicon and oxygen atoms lacking long-range periodicity, hence the term “amorphous” is used to describe its microstructure.
Understanding the basics of chemical bonding, microstructure and electron interactions with optical radiation, it can be understood why some materials are susceptible to UV-degradation and others not. Hence, this discussion will be limited to glass and polymeric materials.
UV damage mechanisms explained
In glass – The dominant mechanism of UV degradation in fused silica is related to the impurities that are inevitably present in the glass, for example, metals such as iron. These metallic atoms have electrons that can be promoted to higher energy levels or freed from the atom so they are available to interfere with electromagnetic radiation, forming so-called “color-centers” and causing a reduction of UV-transparency in the glass over time, which is called solarization. There are also intrinsic atomic defects in silica, unrelated to impurities, such as non-bonded silicon and oxygen atoms that have some absorption in the vacuum-UV (VUV) and UV-C portion of the spectrum. These tend to be more significant in ultra-pure synthetic silica vs. naturally derived quartz glass. As a side-note, when considering the gradual loss of UV-C output from a low-pressure mercury lamp over time, the formation of mercury oxide on the inside surface of the lamp body is a much more significant degradation effect than solarization of the glass.
In polymers – Since most polymers consist of covalently bonded organic constituents, most are susceptible to damage by UV. The most basic and prevalent UV damage mechanism in polymers is called chain scission by photolysis – the breaking of long chains into shorter ones by the direct action of high-energy photons breaking the “backbone” of the molecule. This reduction in molecular weight of a polymer almost always results in a degradation of physical properties such as strength and ductility and a degradation of aesthetic properties such as color and texture. Degradation of polymers also can release by-products into the surrounding environment (e.g., outgassing), which can be problematic for a variety of reasons.
Other UV-induced damage mechanisms in polymers include the formation of radicals – atoms or molecules with unpaired electrons that are highly reactive – when chemical bonds are broken. These radicals will react with other available bonds nearby and cause scission or degradation of the polymer molecules. Bonds dissociated by UV are also prone to reaction with available oxygen or water, usually at the surface of the polymer, causing the degradation mechanisms of oxidation and hydrolysis, respectively. These several degradation mechanisms have been explained separately, but, in fact, they occur in combination and often synergistically. The basic premise is always that the absorption of high-energy UV photons can promote electrons to higher energy levels and dissociate chemical bonds, causing chemical and microstructural changes in the material.
Some familiar examples of UV-induced polymer degradation are the yellowing and “chalking” of PVC pipes installed outdoors, the fading of colors in signage and posters that are exposed to the sun, the chalking and embrittlement in the insulation of wires that are exposed to sunlight or in a UV-C system, and, of course, sunburn. Skin consists of polymers, specifically a protein called collagen. And, if magnified even further, it can be seen that the nuclei of all cells contain long polymer molecules called DNA. The UV-induced damage of DNA is the basis for UV disinfection.
Prevention or mitigation of UV degradation
Understanding the mechanisms of UV degradation gives insight into how to prevent or reduce its effects. The methods of preventing/retarding UV degradation (a.k.a. UV-stabilization) can be separated into several categories:
Inherently UV-resistant polymers
Some polymers withstand UV exposure better than others. The reasons for this are too complicated to elucidate here, but they are related to the above-mentioned aspects of the types of organic bonds that are present. Since C=C double bonds are particularly susceptible to UV photolysis, it makes sense to choose polymers with fewer of these bonds, therefore polyolefins such as polyethylene can be a good choice.
There is a class of high-performance engineering polymers called fluoropolymers that exhibit excellent UV resistance. Common examples of fluoropolymers are polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene (FEP) and polyvinylidene fluoride (PVDF). DuPont’s trademark Teflon has become a genericized name for all fluoropolymers. These polymers derive their exceptional performance from the unique characteristics and strength of the carbon-fluorine bond. In addition to having superior performance and properties such as high-temperature stability, high dielectric strength and extreme chemical inertness, fluoropolymers are exceptionally resistant to UV degradation. Accordingly, PTFE or FEP are almost always used for wire insulation on UV lamps or in UV equipment. Of course, with their high performance comes high prices – fluoropolymers are among the most expensive polymers.
UV-absorbing additives (organic or inorganic)
and radical scavengers
Inorganic additives – As discussed earlier, inorganic compounds are rarely affected by UV exposure. Therefore, it stands to reason that adding inorganic filler to a polymer should help improve UV stability by absorbing UV photons and thereby reducing the damage to polymer bonds. The most common inorganic materials used for UV stabilization are carbon black (soot, essentially) and oxide ceramics, such as aluminum oxide or titanium dioxide. The tradeoff with using such fillers is that they have to be included in relatively high-volume percentages and will alter the physical properties of the polymer as well as its color, though they also can impart other useful properties, such as abrasion resistance. For example, polymers filled with carbon black will necessarily be black in color.
Organic additives – There are numerous categories, including antioxidants, UV absorbers, quenchers and radical scavengers. It would be beyond the scope of this article to identify and explain all these various chemicals, but they generally rely on the following principles for their UV-stabilizing effect:
- UV absorption – These molecules absorb strongly in the UV spectrum and dissipate the photon energy either by turning it into heat or emitting it at longer wavelengths (fluorescence).
- Radical scavenging – These molecules will preferentially react with radicals that are created by photochemical or oxidative changes, thereby neutralizing them before they can do further damage to the polymer chains.
Organic additives can be added to polymers at much lower concentrations than inorganic fillers to achieve the desired UV stabilization. In fact, many such additives also assist in preventing oxidation during high-temperature processing and normal use of the polymer, so they are often added regardless of anticipated UV exposure. However, such additives are expensive, can alter properties and processability of certain polymers, and some are potentially harmful to human health.
Shielding and coating
A simple method of preventing UV degradation of an object is to protect it with a barrier that is impenetrable to UV photons. This could be as simple as shading with a thin layer of aluminum foil or another material that is impervious to UV.
When simple shielding or shading is not possible, an alternative is to apply a coating that absorbs or reflects UV. Many paints contain UV-protecting additives like those described above.
Furthermore, a paint that contains metallic particles can be a very effective UV barrier, although the polymer binders in said paint could themselves be subject to UV degradation. High-performance paints that are used outdoors often contain PVDF and are known for excellent gloss- and color-retention. One could potentially avoid the drawbacks of bulk polymer additives by instead using a UV-stabilizing coating on the surface of the polymer.
Conclusions
It is the author’s opinion that the greatest effect in prevention of UV degradation will occur as a result of following two basic principles:
Good design – that which minimizes the UV exposure of sensitive and critical components through the simple principle of shielding.
Good materials selection – choose suitable and, preferably, inherently UV-resistant materials when UV exposure cannot be avoided.
The latter principle leaves a salient question: What are suitable choices of materials for UV-exposed applications? The available literature on prevention of UV degradation is scattered and difficult to search for or often just lacking. A concise and well-organized handbook for choosing appropriate materials for UV exposure would be a valuable tool in the UV industry and beyond.
It is this question that leads to a recommendation that a task force should be formed dedicated to studying and compiling the body of knowledge on UV-stable – and UV-susceptible – materials as well as performing experimentation where published knowledge is lacking. The goal of said task force would be to publish a list of approved materials shown to hold up well against UV-C degradation and perhaps a “never use” list of ones that are poorly suited for UV applications.
Endnote: The author can find no published evidence of UV degradation of ceramics but would be very interested to hear of any.
Reference
Contact: Chris Rockett, [email protected]
The IUVA task force model is designed to facilitate targeted activities, in lieu of traditional committees. Those interested in participating in an IUVA task force on materials, please contact Gary Cohen at [email protected].
