ADVANCED BIO-FRIENDLY POLYMERS Photo degradation of polymers Štefan Chmela
One of the disadvantages of using polymers in high temperature conditions or in outdoor applications –degradation environment negatively influences the service life. This process is called weathering - ageing an irreversible chemical process, undesired changes of properties of the polymers, discoloration and loss of mechanical properties. outdoor applications -reactions of the polymer with and without oxygen induced by terrestrial sunlight the UV-radiation is one of the most important factors determining the polymers lifetime.
Solar irradiance spectrum above atmosphere and at surface. Sunlight in space at the top of Earth’s atmosphere at a power of 1366 watts/m2 is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.
Ultraviolet Cor (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surface. This spectrum of radiation has antiseptic properties and is used in germicidal lamp. Ultraviolet Bor (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the most of photochemical reactions. Ultraviolet Aor (UVA) spans 315 to 400 nm.
Degradation due to UV-radiation is called photodegradation. Chemical reactions -chain scissions, - cross linking - oxidation influence the physical properties and thus the article's lifetime Besides the service environment, other parameters, as the polymer itself and the use of stabilizers influence the rate of degradation. The most important polymer-related parameters for degradation are the type of polymer, (e.g. polyolefins, engineering plastics as polyamides or polycarbonates), the amount of branching, catalyst residues, or end groups. Different techniques to stabilize polymers have been developed, e.g. adding different types of stabilizers, or applying a protective coating
light is absorbed by a polymer - photochemical reactions can occur as a result of activation of a polymer macromolecule to its excited singlet or triplet states precursors of all photochemical reactions The most important mechanisms causing weathering of polymers are photolysis and photooxidation. If the absorption of light leads directly to chemical reactions causing degradation, this is called photolysis. Photo-oxidation is a result of the absorption of light that leads to the formation of radicals that induces oxidation of the material. oxidation products are distributed non-homogeneously in the sample
photo-oxidation polyolefines (PE, PP) = photo-oxidation is the dominating mechanism. These polymers do not have an inherent absorption at wavelengths present in terrestrial sunlight (>290-400 nm) photolysis can not play an important role. Nevertheless, irradiation of these polymers with terrestrial wavelengths results in accelerated degradation – especially for PP This can be ascribed to impurities that are formed during storage and processing. Due to photolytic reactions of these absorbing species, radicals are formed that initiate the photo-oxidation reaction.
impurities responsible for the initiation • Hydroperoxide >>carbonyls> unsaturations • > complex[PH...O2], atmospheric impurities(SO2, NO), aromatic hydrocarbons, singlet oxygen • Main steps of photo-oxidation • Initiation • Propagation • Branching • Termination
In contrast to polyolefins, the majority of engineering plastics e.g. aromaticpolyesters, polyamids, polyuretanes,polycarbonates,polyketonesetc.) do have absorptions at wavelengths being present in terrestrial sunlight, so that for these polymers photolysis can play an important role too. For these polymers in principle there are three mechanisms that can describe their light-induced degradation: • Photolysis- absorption as a result of the inherent polymeric structure results in chemistry causing changes in the molecular structure; • Photo-oxidation initiated by photolysis reactions of the polymer itself; • Photo-oxidation initiated by impurities not part of the inherent polymer structure.
Photolysis: Norrish I, Norrish II When light is absorbed by the polymer, Norrish reactions can occur, which lead to changes in molecular structure resulting in degradation. The Norrish I reaction leads to chain cleavage and radicals that might initiate the photo-oxidation.
The Norrish II reaction is a non-radical intramolecular process, in which hydrogen (hydrogen on gama C), is transferred, leading to chain cleavage. For polyamides and polyesters (biodegradable polymers) the most important photolytic reactions are the Norrish I and II reactions.
photo-Fries reactions phenyl ester reaction, production of more photoactive ketone photo-Fries rearrangement may occur naturally, for example when a plastic bottle made of polyethylene terephthalate (PET) is exposed to the sun, particular to UV light at a wavelength of about 310 nm.
Radical oxidation process of irradiated PLA samples: hydroperoxide chain propagation and formation of anhydrides by photolysis of hydroperoxide
General scheme for degradation and stabilization
Testing methods Natural ageing Artificial ageing Changes are followed by spectral methods (FTIR, UV-Vis, fluorescence) GPC – molecular mass mechanical properties
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Monitoring of photo-oxidation by FTIR spectroscopy
Changes of FTIR spectra of PP film during irradiation Changes of the carbonylregion Changes of the –OH vibrational region
PP powder fresh IIIIII IV
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