RADIATION DEGRADATION In normal service it is UV radiation which causes degradation Depending on the polymer type two modes of change may be indicated. 1) Reduction in molecular mass Polymer softens and may ultimately become a liquid 2) Cross-linking Polymer embrittles. Combination of 1) and 2) also possible Protection 1) Add UV stabilisers 2) Add pigments 3) Add waxes

INTRODUCTION In a normal environment it is UV radiation which is the major problem. hv A - B (A - B) excited state deexcitation A + B degradation i.e bond scission Such excitations may be specific to given molecular groups

TYPE OF RADIATION Type of radiation Wavelength  rays 5*10E-11 to 5*10E-12 m X rays soft 2*10E-9 to 1*10E-10 m X rays hard 1*10E-10 to 1*10E-11 m U V 4*10E-7 to 1.8*10E-7 m Visible light 4.2*10e-7 to 6.5*10E-7 m IR light 1*10e-5 to 7.5*10E-5 m far 1*10E-4 to 1*10E-5 m i.e. type of radiation determines the energy of the radiation. Radiation excites the molecule and if excitation sufficiently high bonds will break.

EFFECT OF RADIATION EXPOSURE 1- Polymer crosslinks due to side group bond breakages Example:- Polyethylene, Polypropylene, Polystyrene, Polyamides and Rubbers In general, polymer with this chain structure. H ------CH2 – C – X

2 - Polymers which show a reduction in molecular mass. Example:- Polyisobutylene, polymethylmethacrylate, Polymethylstyrene, P.T.F.E., Cellulose. In general, polymer with this chain structure. X ------CH2 – C – Y The tendency to degrade is related to:- a) Absence of a tertiary hydrogen atom. b) A weaker than average main chain C – C bond. c) The presence of unusually strong bonds elsewhere in the molecule e.g. C – F bonds

PROTECTION 1) Add UV stabilizers i.e. molecules which preferentially absorb UV radiation (see antioxidant) 2) Add waxes - Such waxes must have a limited polymer solubility so that they bloom to the surface 3) Pigmentation e.g. carbon black. preferential absorption of UV by the pigment

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