14.08.2014
Light is generally described as the radiation visible to the human eye at wavelengths between 400 and 750 nm. The energy required for photochemical reactions is passed to the molecules in the form of light (absorption). Photo-oxidative degradation processes, in which chain fission, chain branching and oxidation reactions all play a part, can be divided into stages, as shown.
Stabilization The photophysical and photochemical processes that cause photo-oxidative degradation of polymers provide an indication of the best way to protect or stabilize these substances against the harmful effects of light. Incorporation of pigments is probably the oldest way of providing protection against UV light.
The main function of UV absorbers is to absorb UV radiation in the presence of a chromophore (Ch) found in the polymer, the aim being to filter out the UV light that is harmful to the polymer before Ch* has had a chance of forming. Each of these UV absorber groups can be characterized by a typical absorption and transmission spectrum. Extinction depends on wavelength and can be regarded as a measure of the stabilizing or screening effect of the UV absorber. Antioxidants The stability of the phenoxy radical formed according to the equation depends on the substituents R1 to R’ and thus on the possibility of resonance stabilization (delocalization of the electron).
Sterically hindered amines (HALS) Sterically hindered amines have been used to stabilize polymers on a commercial scale since the early seventies. Diagram of Beer–Lambert absorption of a beam of light as it travels through a cuvette of width l. Thus, if the path length and the molar absorptivity (or the absorption cross section) are known and the absorbance is measured, the concentration of the substance (or the number density of absorbers) can be deduced. Although several of the expressions above often are used as Beer–Lambert law, the name should strictly speaking only by associated with the latter two. In molecular absorption spectrometry, the absorption cross section ? is expressed in terms of a linestrength, S, and an (area-normalized) lineshape function, ?. The law tends to break down at very high concentrations, especially if the material is highly scattering. Define z as an axis parallel to the direction that photons of light are moving, and A and dz as the area and thickness (along the z axis) of a 3-dimensional slab of space through which light is passing. We assume that dz is sufficiently small that one particle in the slab cannot obscure another particle in the slab when viewed along the z direction. It follows that the fraction of photons absorbed when passing through this slab is equal to the total opaque area of the particles in the slab, ? A N dz, divided by the area of the slab A, which yields ? N dz.
It is instructive to consider the consequences of error in an assumption that is implicit in this derivation, namely that every absorbing particle behaves independently with respect to the light.
There are at least five conditions that need to be fulfilled in order for Beer’s law to be valid.
The incident flux must not influence the atoms or molecules; it should only act as a non-invasive probe of the species under study.


Beer's law can be applied to the analysis of a mixture by spectrophotometry, without the need for extensive pre-processing of the sample. For a more complicated example, consider a mixture in solution containing two components at concentrations c1 and c2. This law is also applied to describe the attenuation of solar or stellar radiation as it travels through the atmosphere. BMG LABTECH has become a world leader in microplate reading systems due to continuous integration of new and unique technology. Users never have to worry again about setting or tuning different wavelengths for their absorbance assays. Molecules in the singlet ground state, S0, can only be converted into energy-rich states (S1 and T1) by excitation. In intramolecular energy transfer, an excited molecule portion, D*, passes energy to a non-excited molecule portion, A, inside the same polymer molecule.
Titanium dioxide and carbon black are both capable of absorbing UV light and thus help to stabilize paint films. In other words, the higher the extinction, the higher the UV light screening and the greater the stabilizing effect – always assuming that the UV absorber is not itself destroyed by the absorption of the light. To suppress that reaction, the molecules used must be capable of trapping the radicals that have been formed and thereby interrupt the chain reaction. The more stable the phenoxy radical, the less likely it is to initiate further chain reactions. This means, in effect, that after a certain period, depending on the initial concentration and conditions within the polymer, no more antioxidant is left to prevent undesirable free-radical reactions. These substance are almost exclusively derivatives of 2,2,6,6-tetramethylpiperidine and are generally referred to as HALS, which stands for hindered amine light stabilizers. Error is introduced when particles interact by lying along the same optical path such that some particles are in the shadow of others. In particular, this implies that the light should not cause optical saturation or optical pumping, since such effects will deplete the lower level and possibly give rise to stimulated emission. In practice it is better to use linear least squares to determine the two concentrations from measurements made at more than two wavelengths. The spectrometer incorporates a highly-efficient optical grating and a solid state array detector that allows the measurement of light intensity throughout the UV and visible parts of the spectrum. The spectrometer will capture full-absorbance data at a resolution of 1 nm for all wavelengths from 220 to 1000 nm in less than one second per well.
The ultraviolet light that reaches the earth’s surface accounts for only about 6% of all the light reaching it, but that 6% is nevertheless responsible for most of the damage produced when polymers are exposed to weathering.
The transition T1 to T2, T3, etc., is possible only if a molecule that is already in T1 absorbs light a second time. During this reaction, the energy transfer from a photo-activated donor, D*, to an acceptor, A, which is present in the ground state, plays an important part.


However, these data need to be modified to allow for possible impurities, which are unavoidable in industrially produced polymers, as well as additives, pigments, extender pigments or even dyes. Electron spin resonance (ESR) has shown that, under photo-oxidative conditions, HALS are largely transformed into the corresponding stable nitroxyl radical. Mixtures containing more than two components can be analysed in the same way, using a minimum of n wavelengths for a mixture containing n components. In essence, it is our customers that drive innovation and advancement of our microplate readers.
The transitions from S1 to T1 and from T1 to S0 break the spin selection rules (change of spin multiplicity). Accordingly, the UV absorber should also be able to absorb light at higher wavelengths, without adversely affecting the color of the cured coating. For a UV absorber to be effective, it must absorb UV light better and faster than the polymer it is meant to stabilize and dissipate the absorbed energy before unwelcome side reactions are triggered. BMG LABTECH is the only microplate reader company in the world offering spectrometer technology in a multi-detection microplate reader. Reactions in the shorter-lived singlet state, S1, occur if they are thermodynamically and kinetically possible.
However, titanium dioxide is available in various forms, namely anatase (treated or untreated) and rutile (treated or untreated). The purpose of UV absorbers is to absorb harmful UV light and quickly transform it into harmless heat.
This means that transformation of the energy absorbed in the form of UV light must take place in the singlet state.
The smaller the energy difference between S1 and T1, the greater the possibility of inter-system crossing. Titanium dioxide can initiate polymer degradation, depending on the way it has been modified and treated, to form hydroxyl and hydroperoxide radicals. During this process, absorbed energy is converted into vibrational and rotational energy of the molecule constituents. Inter-system crossing (transition S1 to T1) and therefore phosphorescence must be excluded.
A molecule in the excited singlet state, S1, has more deactivation opportunities, as shown below in the Jablonski diagram. For UV absorbers to be effective, it is essential that this process take place more rapidly than the corresponding reaction within the substrate, and that neither the UV absorber nor the polymer it is intended to stabilize are damaged during energy conversion.
Under these conditions, the detectability of a given technique is often quoted in terms of ppm•m.



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