Plastic surfaces, reflectance spectroscopy

For a discussion of reflectance spectroscopy, two types of reflectance must be defined, specular and diffuse. Specular reflectance is simply mirrorlike reflectance from a surface and is sometimes called regular reflectance it has a well-defined reflectance angle. Diffuse reflectance is defined as reflected radiant energy that has been partially absorbed and partially scattered by a surface with no defined angle of reflectance. The diffuse reflectance technique is widely used today for industrial applications involving textiles, plastics, paints, dyestuffs, inks, paper, food, and building materials. In the area of basic research, diffuse reflectance spectroscopy has been used in studies of solid-solid reactions, of species absorbed on metal surfaces, of radiation transfer, and of slightly soluble species. 

Visible reflectance spectroscopy is used routinely to measure the colour of opaque objects such as textile fabrics, paint films and plastics for purposes such as colour matching and dye and pigment recipe prediction. There is now a wide range of commercially-available reflectance spectrophotometers used industrially as colour measurement devices for such purposes. In many ways, this technique may be considered as complementary to the use of visible absorption spectroscopy for the measurement of transparent dye solutions. Reflectance spectra of typical red, green and blue surfaces... 

Two small extraction disks are placed one on top of the other in a plastic holder. Then a syringe containing the aqueous sample is attached to the holder and the sample is pushed through the disk at a rate of about 4—5 mb min . AU of the chromium(VI) is retained on the top anion-exchange disk and the chromium(III) is extracted by the second cation-exchange disk. The concentrations on each disk are several hundredfold higher than they were in the original sample. The amounts of chromium(IIl) and (VI) extracted were measured directly on the surface of the respective disks by diffuse reflectance spectroscopy (DRS). 

Internal reflectance spectroscopy (IRS alternatively named attenuated total reflectance, ATR) is a quick and easy non-destructive sampling technique for obtaining the IR spectrum of a material s surface or of material which is either too thick, or strongly absorbing, to be analysed by more traditional transmission methods, cfr Chp. 1.2.1.4. Internal reflection techniques, which require close contact with an internal reflection element (IRE) are unsuitable for rapid screening of plastic materials. 

The most widely available technique for identifying mainly polymer, but also additives in plastics, is Fourier Transform Infrared (FTIR) spectroscopy. Samples are exposed to infrared light (4000-400 wavelengths per centimetre or cm ) causing chemical bonds to vibrate at specific frequencies, corresponding to particular energies. In the last 5 years, an accessory for FTIR has been developed, which enables non-destructive examination of surfaces and so is ideal for analysis of plastics in museum collections. Attenuated Total Reflection-FTIR (ATR-FTIR) requires samples to be placed on a diamond crystal with a diameter of 2 mm through which the infrared beam is reflected... 

Infrared spectroscopy (IR) is used extensively in the analysis of plastics. It is a very good, relatively quick technique for the determination of the polymer present in a plastic product. Unlike rabbers, where the majority of compounds contain additives such as carbon black and plasticisers that can interfere with the infrared data obtained, it is normally possible to record infrared spectra directly from the plastic sample using either a surface technique (e.g., attenuated total reflectance (ATR) spectroscopy) or in transmission through the sample from a film produced by pressing a specimen above its glass transition or melting point. 

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