Photocatalytic Reactions
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron-hole pairs, which generate free radicals (hydroxyl radicals: .OH) able to undergo secondary reactions. Its comprehension has been made possible ever since the discovery of water electrolysis by means of the titanium dioxide. Commercial application of the process is called Advanced Oxidation Process (AOP). There are several methods of achieving AOP's, that can but do not necessarily involve TiO2 or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl radical
The principle of photocatalytic reaction was to accelerate the nature's cleaning and purifying process using light as energy. Discovered in 1960's, Dr. Fujishima of Japan found titanium metal, after irradiated by light, could break water molecule into oxygen and hydrogen gas. By restructuring titanium dioxide particles in nano-scale, a number of new physical and chemical properties were discovered. One of these newfound effects was photocatalytic oxidation which accelerated the formation of hydroxyl radical, one of the strongest oxidizing agents created by nature. Using energy found in the UV light, photocatalyst titanium dioxide could breakdown numerous organic substances such as oil grime and hydrocarbons from car exhaust and industrial smog, volatile organic compounds found in various building materials and furniture, organic growth such as fungus and mildew. In addition to its photocatalytic oxidation effect, titanium dioxide coating also exhibited hydrophilic property (or high water-affinity) which titanium dioxide coating attracted water moist in the air to form an invisible film of water. This thin film of water allowed the substrate to be anti-static so the coated surface could be easily cleaned by rinse of water. For years, titanium dioxide was used in many commodity products such as paint, cosmetics, sun blocks, and etc. It is a safe and stable substance commonly found in our lives. Numerous applications have been developed from utilizing photocatalytic reaction.
When photocatalyst titanium dioxide (TiO2) absorbs Ultraviolet (UV) radiation from sunlight or illuminated light source (fluorescent lamps), it will produce pairs of electrons and holes. The electron of the valence band of titanium dioxide becomes excited when illuminated by light. The excess energy of this excited electron promoted the electron to the conduction band of titanium dioxide therefore creating the negative-electron (e−) and positive-hole (h+) pair. This stage is referred as the semiconductor's ‘photo-excitation’ state. The energy difference between the valence band and the conduction band is known as the ‘Band Gap’. Wavelength of the light necessary for photo-excitation is: 1240 (Planck's constant, h)/3.2 ev (band gap energy)=388 nm
The positive-hole of titanium dioxide breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative-electron reacts with oxygen molecule to form super oxide anion. This cycle continues when light is available
Photocatalytic oxidation is achieved when UV light rays are combined with a TiO2 coated filter. TiO2 refers to Titanium Dioxide. This process creates hydroxyl radicals and super-oxide ions, which are highly reactive electrons.
These highly reactive electrons aggressively combine with other elements in the air, such as bacteria and VOCs. VOC is an acronym for Volatile Organic Compounds which include harmful pollutants such as formaldehyde, ammonia and many other common contaminants released by building materials and household cleaners generally found in the home. Effective oxidation of the pollutants breaks down into harmless carbon dioxide and water molecules, drastically improving the air quality.
Biopolymers
With growing environmental concerns over petrochemical products and its environmentally harmful practices, new environmentally friendly polymers being developed as a replacement for petrochemical based plastics. Materials such as PLA (polylactic acid) such as product produced by Natureworks (Cargill) are derived from natural and rapidly renewable resources of corn. To date the vast majority of interest and commercialization is the application of PLA for disposable packaging and other disposal products. Although thought of as a disposable plastic, PLA has many new abilities and functions that can further expand the usage of this environmentally friendly biobased technology.
Polylactic acid is not derived from petrochemical materials, but from the conversion of starch or cellulosic materials into dextrose then into a lactic acid. The lactic acid is then polymerized into a range of polymer products. This conversion process has been documented and is currently commercialized. Being that PLA is not petrochemical based, it has other unique functional and processing abilities outside that of petrochemicals that provides unique optical and functional properties outside of the needs for basic disposable packaging.
Plastics typically block UV such as acrylic, polystyrene, PE, PP and most all petrochemical plastics. Currently fused quartz mineral is used for UV transparent applications, but is both difficult and expensive to shape or form into shapes. Secondly quartz mineral cannot be easily soften to fuse nanominerals onto its surface. Currently few materials are UV transparent and most are expensive or classified as a hazardous material. Traditionally material such as quartz or sapphire have been used in some of these industries providing a high degree of UV stability. These material have limitations in cost, fabrication, and other limitations. Other engineered polymers such as fluoropolymers have been used in UV transparent applications, but are hindered by cost and health considerations. Law suits have been won suing company's based on fluoropolymers emissions and pollution.
PLA is a thermoplastic polyester derived from field corn of 2-hydroxy lactate (lactic acid) or lactide. The formula of the subunit is: —[O—CH(CH3)—CO]— The alpha-carbon of the monomer is optically active (L-configuration). The polylactic acid-based polymer is typically selected from the group consisting of D-polylactic acid, L-polylactic acid, D,L-polylactic acid, meso-polylactic acid, and any combination of D-polylactic acid, L-polylactic acid, D,L-polylactic acid and meso-polylactic acid. In one embodiment, the polylactic acid-based material includes predominantly PLLA (poly-L-Lactic acid). In one embodiment, the number average molecular weight is about 140,000, although a workable range for the polymer is between about 15,000 and about 300,000. In one embodiment, the PLA is L9000™ (Biomer, Germany), apolylactic acid)
Polylactic acid is a relatively high specific gravity as compared to common plastics with a specific gravity closer to engineered plastics such as Polycarbonate. Although similar in specific gravity to polycarbonate used in various functional arid optical products, PLA has a much lower refractive index. In addition due to the unique molecular structure and materials, PLA is virtually transparent in UV wavelength spectrum as compared to polycarbonate and other common plastics that have very high UV absorption rates. From this PLA does not have visible or UV degradation or yellowing as found in common plastics. UV transparency and a low refractive index can have a myriad of applications.
UV Resistance and UV Transparency
It has been discovered the PLA has very good UV resistance in regards to UV degradation. Various tests have been performed in UV weatherometers showing that PLA does not yellow when exposed to exterior light. In addition, tests based on UV-visible photospectrometers show that PLA is transparent the UV A, UV B, and in most of the UV C ranges. This shows that the material allows full transmission of UV waves.
Other materials such as polycarbonate have high degrees of clarity in the visible light spectrum but have high degrees of UV absorption. Most polymers are carefully measured for their UV absorption due to the fact that the absorption of UV has a significant relationship to UV degradation of the polymers. Polymers are vary greatly in their resistance to weathering, such as polymethylathacrylate (PMMA) and polytetrafluosoethylene (PTFE) are transparent to UV radiation and hence not susceptible to photodegradation. Materials such as PTFE and PMMA are considered “UV Transparent” materials
According to data obtained, the following show a specific wavelength wherein the material starts to absorb UV-visible wavelengths:
PET420 nmPolycarbonate330 nmPLA240 nm
UV or ultra violet radiation is a shorter wavelength than visible light spectra. The following represents the areas of various UV energy classifications:
UV ALong wave (black light)315 to 400 nmUV BUB Medium wave280 to 315 nmUV CShort wave (germicidal)100 to 280
From the above chart reference, it can be seen that PLA starts absorption at a much shorter UV wavelength and in addition the amount of absorption is lower than that of a high quality PET that significantly lower than a polycarbonate material.
PLA also is unique in the fact that it has a high surface energy. PLA has a similar range of refractive index as Fluoropolymers, but with much higher surface energy.
Little work has been found in the areas of measurement of various optical, electrical or other functional performance of PLA and various methods of hybridizing PLA with the addition or various additives, chemicals or nanomaterials.
Polylactic acid has a specific gravity typically around the 1.25 range and can produced in a transparent form. Common plastics for optical and other functional applications such as polycarbonate have specific gravities of typically 1.2 to 1.22.
The optical nature of petrochemicals is known and used for many applications including eyewear lens, television display screen s, protective coatings and myriad other optical applications.
Optical properties such as refractive index, UV absorption/transmission and UV resistance are important issues related to optical properties within petrochemical polymers used in optical applications.
Refractive Index
The refractive index or index of refraction is a ratio of the speed of light in a vacuum relative to that speed through a given medium (this quantity does not refer to an angle of refraction, which can be derived from the refractive index using Snell's Law). In other words, as light passes from one medium to another as from air to water, the result is a bending of light rays at an angle. This physical property occurs because there is a change in the velocity of light going from one medium into another. Refractive index also describes the quantity that light is bent as it passes through a single substance. This involves calculating the angle at which light enters the medium and comparing that with the angle at which the light leaves the medium.
Another view rates each substance with its own refractive index. This is because the velocity of light through the substance is compared as a ratio to the velocity of light in a vacuum. The velocity at which light travels in a vacuum is a physical constant, and the fastest speed at which energy or information can travel. However, light travels slower through any given material, or medium, that is not a vacuum. This is actually a delay from when light enters the material to when it leaves; i.e., when some is absorbed, and another part transmitted. The following shows various refractive indices of plastics:
Specific GravityRefractive IndexPolycarbonate 1.2-1.221.58Polylactic Acid 1.24-1.251.46 note: Range withblending (1.4 to 1.55)
The difference of refractive index between PLA and conventional petrochemical polymers also provides other potential functional features including electrical dielectric strength.
The dielectric constant (which is often dependent on wavelength) is simply the square of the (complex) refractive index in a non-magnetic medium (one with a relative permeability of unity). The refractive index is used for optics in Fresnel equations and Snell's law; while the dielectric constant is used in Maxwell's equations and electronics
Thus from this basic physics the dielectric constant of PLA would be lower than conventional petrochemical plastics and have various applications in electrical components and systems.
Fluoropolymers have been investigated for a wide range of innovative optical applications not only because of their possible optical clarity but also because their refractive indices are generally much lower than competing materials such as PMMA and PC. The refractive index for most fluoropolymers is in the region of 1.30 to 1.45 compared with the refractive index for more traditional transparent polymers such as PMMA and PC where it is in the region of 1.5 to 1.6 (or higher). This makes the fluoropolymers suitable for optical technology products such as waveguides, optical filters, fiber gratings and a wide range of optical devices. Specialist ultra-transparent fluoropolymers are also being developed for these applications and for use in rapidly developing CMOS lithography technologies essential for the production of semiconductor devices. The optical clarity and other performance properties of fluoropolymers are opening new markets and opportunities.
The usage of dissimilar materials with various refractive indexes are used for a wide range of applications for antireflective coatings, LCD flat panel screen assemblies, general optical lensing and other similar applications. A lower or different refractive index of PLA in combination with a convention higher refractive index can have unique applications and provide a tool for design of new optical based systems.
Luminous Transmittance
Luminous transmittance for various materials is provided below.
Optical glass99.9PMMA92PC89SAN88PS88ABS79PVC76