Light is generally described as the radiation visible by the human eye on wavelengths of 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 in below. Most of the energy-rich, electromagnetic radiation (<290 nm) is absorbed by the earth’s atmo-sphere, especially by the ozone layer in the stratosphere.

The ultraviolet light that reaches the earth’s surface represents only about 6% of all the light reaching it, but that 6% is nevertheless respon-sible for most of the damage produced when polymers are exposed to weathering influences.

Only light that is absorbed is capable of initiating photochemical processes. Pure polymers such as polymethyl methacrylate, polyethylene or aliphatic polyesters are photochemically stable between 300 and 400 nm because they do not absorb light. The emphasis here is on the word “pure”, which means that, if the polymer contains impurities which absorb light, e.g. catalyst residues and other substances added during production, or oxidation products, it becomes sensitive to UV light. Polymers whose basic structure already contains 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 done to polymers and paint films 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 here 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)

According to 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 the lifetime of the state. 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 possibilities of deactivation, 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 which 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 that 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 those molecules (e.g. copolymers) in which a part can readily be photoactivated.

Stabilization
The photophysical and photochemical processes which 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.

UV-absorbent pigments
Incorporation pigments are 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, of course, be used because of their color. It should be remembered that pigments such as titanium dioxide can also cause photo-oxidative degradation of polymers. It should however be noted that 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 which, although capable of absorbing UV light, otherwise remain “invisible” and cause no undesirable side effects.

UV absorbers
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 in the region between 290 and 350 nm. If, however, one takes possible impurities into account, which are unavoidable in industrially produced polymers, as well as additives, pigments, extender pigments or even dyes, this information has to be modified. Accordingly, the UV absorber should also be able to absorb 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 takes place more rapidly than the corresponding reaction within the substrate, and that neither the UV absorber nor the polymer it is meant 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, above all, by the Lambert-Beer Law

Extinction depends on the wavelength and may be regarded as a measure of the stabilizing or screening effect of the UV absorber. In other words, the higher the extinction, the more UV light is screened 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
• it must 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 T1) and therefore phosphorescence must be excluded.

Free-radical scavengers
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, molecules have to be used that are capable of trapping the radicals which have been formed and thereby interrupt the chain reaction. Such substances are called free-radical scavengers. The most important of these are antioxidants and sterically hindered amines (HALS).

- Antioxidants

The stability of the phenoxy radical formed according to 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 that it will initiate further chain reactions.

Phenolic antioxidants are used especially as stabilizers against 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 since the early seventies to stabilize polymers on a commercial scale. These substance are almost exclusively derivatives of 2,2,6,6-tetramethylpiperidine and are generally referred to as HALS, which is an acronym for hindered amine light stabilizers. ESR (electron spin resonance) has shown that, under photo-oxidative conditions, HALS are transformed into the corresponding stable nitroxyl radical, at least to a large extent.