Influence of transition metal-doped titanium(IV) dioxide on the photodegradation of polyethylene

The rate of photodegradation of polyethylene is reduced by adding certain grades of TiO2 such as coated TiO2 particles or TiO2 doped with small percentages of Cr or Mn ions. The rate is increased on adding TiO2 doped with V and especially Mo or W ions. The anatase form of TiO2 is more photoactive than the rutile form, and the effect of increasing the calcination temperature of the pigment is to reduce photoactivity by boosting the rutile fraction. The concentration dependences of the degradation rates are complex, but can be directly related to the percentage of anatase achieved after calcination. Even the most aggressive of the metal-doped pigments are less photoactive than the Degussa P25 material, containing both rutile and anatase.