background image
process at high temperature (e.g. 180 C for PVC, 220 C for the PP);
high shear stress during the process (e.g. injection of the PP with 240 C);
light
ionizing radiations
metals, residues of polymerization catalysts
critical conditions
critical conditions
radicalar mechanism
radicalar mechanism
(thermal oxidation for polyolefines and PVC)
RH
R
initiation = alkyl R
R + O
2
ROO
conversion in peroxi ROO
ROO + RH
ROOH + R
propagation, hydroperoxide ROOH
ROOH
RO + HO
RO = alcoxy
2 ROOH
RO + ROO + H2O
2 RO
ROOR
peroxide
ROOR
2 RO
DEGRADATION OF THERMOPLASTICS
SOLUTION: ANTIOXYDANTS
+heat
Initiation
PH (polymer)
+ O
2
+PH
carbonyled products
chain breaking
PO + HO
POOH
POO
P
if present
reactions with secondary
antioxidants
(phosphite, thioether)
if present
reactions with
primary
antioxidants
(phenolic)