5.2 Photodegradation and stabilization of polymers

Transcription

1 5.2 Photodegradation and stabilization of polymers Photodegradation of polymers b) Exposure to sunlight and some artificial lights can have adverse effects on the useful life of plastic products. Photodegradation takes place when UV or vis radiation breaks down the chemical bonds in a polymer, ultimately causing cracking, chalking, color changes and the loss of physical properties. In all polymers produced industrially impurities are present. These could be additives and pigments added on purpose, as well as metallic catalysts and residues of photoinitiators. Peroxides, hydroperoxides and ketones chromo-phores can be formed during the thermal processing of the polymer in air. Typical reactions are summarized here on the left. These impurities often absorb more solar light than the intrinsec chromophore moieties carried by the polymer Photodegradation of polymers Intrinsec chromophores contained in polymers and their spectral data

2 Photodegradation of polymers haracteristics of the photodegradation of some polymers Primary photodegradation processes omolytic chain fission 2 3 Φ = Φ = 10 3 b) c)

3 Primary photodegradation processes d) Fission of alkoxy radicals yields ketones, which are new chromophores. A Intramolecular excitation energy transfer (exciton migration along the polymer chain) can produce the fission of a bond rather far from the chromophore primarly excited by light. The same phenomenon can funnel excitation energy on a particular weak acceptor point, producing an antenna effect Photodegradation of polymers Light absorption by ketone chromophores can induce bond cleavage by either Norrish type I or type II mechanisms. In Norrish type II mechanism, the close approach of the excited carbonyl group to a atom, forming a 6 membered ring is a prerequisit and, thus, depends on the polymeric chain flexibility. Quantum yields of main-chain fission of some copolymers (λ = 313 nm) 153

4 Secondary photodegradation processes 2 reaction of radicals with oxygen b) chain reaction c) 2 2 hydrogen abstraction c) termination d) In the absence of oxygen a cleavage of polymeric chains can be observed. More often, however, photodegradation leads to the cross-linking of the polymer eactions due to dyes and pigments Singlet oxygen sensitization by dyes S* b) Photo-oxydation by semiconducting pigments Ti2 UV e cb h vb e cb h vb heat e cb 2 2 hydroperoxides h vb polymer oxydation products Fine chalky powder build-up on the surface of polymers (paintfilm) due to pigment release

5 Photostabilization of polymers Because no polymers exist, which are insensitive to prolonged exposure to terrestrial sunlight, they must be stabilized for commercial application. Efforts toward photostabilization of polymers use the following complementary strategies: Prevention of light absorption by polymer-bonded chromophores and impurities eactivation of excited states of chromophores through fast quenching Provision of alternative chemical pathwaysfor peroxides and hydroperoxides Trapping of radicals via additional combination reactions These concepts have lead over the years to four types of stabilizers, often used in combination: UV screeners b) UV absorbers c) Excited state quenchers d) Free radical scavengers and/or hydroperoxide decomposers Photostabilizers UV screeners arbon black, Zn, Ti2, Mg, a3, BaS4, Fe23,... b) UV absorbers c) Quenchers Δ Typical stabilizers acting as UV absorbers are phenyl salicylates, 2- hydroxybenzo-phenones, coumarin derivatives, resorcinol esters,... The hydroxy products dissipate the excitation energyvia reversible transfer in a six-membered transition state, shown here for 2- hydroxybenzophenone. The main effect of quenchers is to deactivate the relatively long-lived excited states of ketones and aldehydes. Excited state s of the quencher formed by intermolecular energy transfer then dissipates the energy non-radiatively and convert it into heat. The stabilizing effect of metal chelates, such as Ni (sees table on next slide) derives from this principle

6 Photostabilizers d) adical scavengers/ peroxides decomposers N N N ALS N Photostabilizers Some commercial photostabilizers

7 Photostabilizers Effect of stabilizers on the photostability of poly(propylene) here on the right table on previous slide