Light is generally described as the radiation visible to the human eye at wavelengths between 400 and 750 nm. This “visible” light forms part of the electromagnetic radiation to which the earth is exposed.
Electromagnetic radiation can be divided into different groups, as shown below. Most of the energy-rich, electromagnetic radiation (<290 nm) is absorbed by the earth’s atmosphere, especially the ozone layer in the stratosphere.
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.
Only light that is absorbed is capable of initiating photochemical processes.
Pure polymers, such as polymethyl methacrylate, polyethylene and aliphatic polyesters, are photochemically stable between 300 and 400 nm because they do not absorb light. The emphasis here is on the word “pure”, because if the polymer contains impurities that absorb light, e.g., catalyst residues and other substances added during production, or oxidation products, it becomes sensitive to UV light. Polymers that already contain UV-absorbent groups are likely to be photochemically degraded, even in the absence of such impurities. Typical examples include polystyrene and styrene copolymers, aromatic polyurethanes, polyesters and polyepoxides.
The damage to polymers and paint films caused by light can be described as a kind of “sunburn”, the difference being that polymers, unlike the human skin, are not capable of regeneration.
The energy required for photochemical reactions is passed to the molecules in the form of light (absorption). The molecules are transformed into an energy-rich, excited state.
Two kinds of electron states can occur in molecules:
Singlet state S (paired electron spins)
Triplet state T (unpaired electron spins)
As described in Hund’s first law, electron states with greater spin multiplicity are more stable, i.e., the triplet state is generally lower in energy than the corresponding singlet state.
Molecules in the singlet ground state, S0, can only be converted into energy-rich states (S1 and T1) by excitation.
The probability of a chemical reaction in the excited state increases with its duration. The lifetime of the excited triplet state, T1, is longer than that of the corresponding singlet state, S1, i.e., most photochemical reactions take place in the excited triplet state. Reactions in the shorter-lived singlet state, S1, occur if they are thermodynamically and kinetically possible. The kinetic factor in particular is dependent on the substrate.
A molecule in the excited singlet state, S1, has more deactivation opportunities, as shown below in the Jablonski diagram.
Deactivation is possible with or without radiation. The lowest triplet state, T1, is formed by a radiation-less transition from S1 to T1, described as “inter-system crossing”. The transition T1 to T2, T3, etc., is possible only if a molecule that is already in T1 absorbs light a second time. The transitions from S1 to T1 and from T1 to S0 break the spin selection rules (change of spin multiplicity). Depending on the strength of the spin orbit coupling, transition is now possible. The smaller the energy difference between S1 and T1, the greater the possibility of inter-system crossing.
Photochemical degradation processes
Photo-oxidative degradation processes, in which chain fission, chain branching and oxidation reactions all play a part, can be divided into stages, as shown. Here, chain initiation is of considerable importance. 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.
This energy transfer can take place inter- or intramolecularly. In intramolecular energy transfer, an excited molecule portion, D*, passes energy to a non-excited molecule portion, A, inside the same polymer molecule. This process is important in molecules (e.g., copolymers), a part of which can readily be photoactivated.
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. Titanium dioxide and carbon black are both capable of absorbing UV light and thus help to stabilize paint films. Pigments cannot be used because of their color. Pigments such as titanium dioxide can also cause photo-oxidative degradation of polymers. However, titanium dioxide is available in various forms, namely anatase (treated or untreated) and rutile (treated or untreated). Titanium dioxide can initiate polymer degradation, depending on the way it has been modified and treated, to form hydroxyl and hydroperoxide radicals.
Pigments can act as UV absorbers only under certain conditions. This is why attempts were made to find other molecules that, although capable of absorbing UV light, otherwise remain “invisible” and cause no undesirable side effects.
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. Above all, a UV absorber must function within the 290 and 350 nm range. 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. Accordingly, the UV absorber should also be able to absorb light at higher wavelengths, without adversely affecting the color of the cured coating.
The purpose of UV absorbers is to absorb harmful UV light and quickly transform it into harmless heat. During this process, absorbed energy is converted into vibrational and rotational energy of the molecule constituents. 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. The most important UV absorbers are: a) 2-(2-hydroxyphenyl)-benzotriazoles b) 2-hydroxy-benzophenones c) hydroxyphenyl-s-triazines d) oxalanilides
Each of these UV absorber groups can be characterized by a typical absorption and transmission spectrum.
The effectiveness of UV absorbers is determined not only by their
absorption characteristics but also, above all, by the Lambert-Beer Law.
Extinction depends on wavelength and can be regarded as a measure of the stabilizing or screening effect of the UV absorber. 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. Extinction thus depends on the extinction coefficient, the concentration, “c”, of the UV absorber in the polymer, and on the film thickness, “d”, of the unpigmented polymers.
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. This means that transformation of the energy absorbed in the form of UV light must take place in the singlet state. Inter-system crossing (transition S1 to T1) and therefore phosphorescence must be excluded.
The excited chromophore, “Ch*”, can either decompose to form radicals, which can then react with the polymer and/or atmospheric oxygen, or remove a hydrogen atom from the polymer and thereby initiate a free-radical reaction. To suppress that reaction, the molecules used must be capable of trapping the radicals that have been formed and thereby interrupt the chain reaction. These substances are called free-radical scavengers. The most important are antioxidants and sterically hindered amines (HALS).
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). The more stable the phenoxy radical, the less likely it is to initiate further chain reactions.
Phenolic antioxidants are used especially to inhibit thermo-oxidation where high processing temperatures are prevalent.
One important drawback of phenolic antioxidants is their non-cyclic mode of action. 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. In order to achieve a long-term effect, the free-radical scavenger should remain effective for an almost unlimited period, i.e., its mode of action should be cyclic.
Sterically hindered amines (HALS)
Sterically hindered amines have been used to stabilize polymers on a commercial scale since the early seventies. 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. Electron spin resonance (ESR) has shown that, under photo-oxidative conditions, HALS are largely transformed into the corresponding stable nitroxyl radical.