Unique gas analyser helps to characterise photoactive pigments
Researchers in Germany have employed advanced FTIR gas analysis to evaluate the photoactivity of pigments in paints and coatings when subjected to light. The Gasmet PCM (Photochemistry Monitoring System) has been developed by Ansyco GmbH, a Gasmet Group Company, to meet the demands of industries that seek to exploit the beneficial characteristics of photocatalytic materials without affecting other performance criteria. The Fraunhofer Institut für Produktionstechnik und Automatisierung (Fraunhofer Society is Europe’s largest application-oriented research organisation) for example, has successfully employed the Gasmet PCM to help evaluate potential materials for a wide range of industrial clients. “We can now offer our customers a well-defined, repeatable procedure to characterise photocatalytic activity. Due to the flexibility of the Gasmet PCM, we are also able to respond to specific requests such as the determination of the influence that various unusual substances might have, and of their degradation products,” comments Fraunhofer’s Dr Matthias Wanner.
The Gasmet PCM is marketed in the UK by Milton Keynes based Quantitech.
Titanium dioxide is the most widely used white pigment in the world because of its brightness and very high refractive index; imparting whiteness, brightness, and opacity when incorporated into a plastic product. Typical applications include paints, varnishes, paper, plastics, printing inks, fibres, rubber, cosmetic products and foodstuffs. However, in addition to its physical properties, some forms of titanium dioxide are also photocatalysts under either visible or UV light, which means that they can be used in modern air and water purification techniques employing photo-oxidation of toxic substances such as volatile organic chemicals (VOCs), polyaromatic hydrocarbons and nitrogen oxides. This process also produces hydroxyl radicals which are known to attack airborne pathogens such as bacteria and viruses, and will therefore find application in healthcare scenarios such as hospitals, clinics and care homes.
In order to develop commercially viable coatings that exhibit the beneficial properties outlined above, it is necessary to be able to assess both the capability of a material to ‘treat’ harmful compounds, and to be able to measure the gases that result from these photochemical reactions. For this reason, FTIR gas analysis technology was chosen for this application because of its ability to measure multiple compounds simultaneously.
Research work to-date has focused on the modification of optical and photocatalytic properties of metal oxides for deployment in water and air purification under visible light irradiation. The photocatalytic properties of active titanium dioxide, for example, are excellent under UV light, but since natural light only contains about 5% UV, much of the research work has sought to find ways to maximise photochemical activity under natural light conditions. As a result, new products such as paints and concrete coatings are being developed to actively reduce VOC and nitrogen oxides levels in ambient air.
These products are designed for locations in which air quality is of greatest concern – in tunnels and roadsides in urban areas for example, where nitrogen oxides are a particular concern. Emissions from vehicles, and especially diesel engines, result in high concentrations of NOx. As a result, many of the world’s major cities exceed limits established by the European Union and others, and these high levels of pollution are responsible for enormous numbers of premature deaths. In the UK for example, the government has estimated that there are around 29,000 premature deaths every year as a result of air pollution and the World Health Organisation reported that ‘in 2012 around 7 million people died - one in eight of total global deaths – as a result of air pollution exposure. This finding more than doubles previous estimates and confirms that air pollution is now the world’s largest single environmental health risk.
Practical application of the GASMET PCM
Dr Wanner says: “The main focus of our work with the Gasmet PCM is the determination of the photocatalytic activity of pigments, either with respect to the model solvent isopropanol, or with respect to representative matrices of the binders typically used in paints and coatings. The determination and quantification of photocatalytic properties are always performed under precisely controlled conditions, namely: relative humidity, oxygen level, temperature, and illumination.
“Additionally, the Gasmet PCM is employed to estimate the weathering resistance of coatings with unprecedented efficiency. Such characterization is basically a prolonged closed-loop measurement during which we are able to determine the photocatalytic passivity of paints and coatings exposed to UV radiation under simulated humid ambient conditions. Under these conditions, the accumulated CO and CO2 concentrations originating from the degeneration of binders can successfully be used to estimate the weathering resistance of coatings.”
Further emphasising the importance of the closed-loop system Dr Wanner says: “We are now able to conduct continuous measurements from the beginning to the end of the test cycle. These measurements include: the remaining concentrations of the model solvents, all the intermediate degeneration products, and the end-products such as CO and CO2. Typically, there are several phases in the degeneration process when the intermediate components are formed and further oxidised. This system enables us to characterise every step of the process and every reaction in a more detailed way than would be possible with an open-loop system.”
The Gasmet PCM
Initially developed to study the photoactivity of titanium dioxide surfaces, the PCM is a versatile research tool for studying various photochemical and surface catalysed reactions with gaseous reactants and products. The entire photocatalytic process, including the quantification of degradation, is monitored with the GASMET FTIR gas analyser. Designed to accommodate several types of surface specimens and light sources in the UV and VIS regions, the PCM incorporates a high-precision gas generator for both humidity and volatile organic compounds. Typical materials tested with the PCM include pigment powders, coated surfaces and photocatalytic air purification materials.
The system consists of:
gas generator and calibration system
advanced sampling system with stainless steel tubing and Metal Bellows® pump
CX4000 FTIR multicomponent gas analyser
The photoreactor is a flow-through metal vessel, a quartz window, illumination source, and a specimen holder for the catalytic material which is irradiated with visible or UV light. The gas generator mixes carrier gas and accurately infused liquid in a heated vaporisation chamber and feeds the mixture into the system. The advanced sampling system circulates the sample gases from the reactor to the FTIR gas analyser and back.
The CX4000 gas analyser used in the Gasmet PCM incorporates a Fourier transform infrared spectrometer, a temperature controlled sample cell, and signal processing electronics. The analyser is delivered as a complete working system and offers versatility with high levels of accuracy and reliability.
The Gasmet CX4000 was originally developed for continuous monitoring of gas concentrations in processes and industrial emissions, and is ideal for analysing trace concentrations of pollutants in hot, wet, corrosive gas mixtures. Calibration is simple, using only single component calibration gases, and users can easily configure the analyser for a new set of compounds.
The development of the Gasmet PCM is a good example of the ways in which FTIR technology can be exploited in a wide variety of applications. In common with all of its applications, Gasmet FTIR is the preferred technology because of its ability to deliver accurate reliable concentration readings simultaneously for multiple compounds from samples that some might consider to be harsh or aggressive. This technology also offers users the ability to identify unknown compounds. The results of the PCM analysis are clearly extremely important and Dr Wanner says: “There is no comparable equipment on the market. For our main client, one of the major advantages of this process is the drastic reduction in time taken to characterise materials in comparison with typical weathering tests. For them, the data generated by the PCM are the main determining factor in the decision as to whether a pigment is introduced to the market.”