Download Stabilization of polymers

ADVANCED BIO-FRIENDLY POLYMERS
Thermal, antioxidative and
photochemical stabilization of polymers:
low molecular weight versus
macromolecular stabilizers
György Kasza
Industrial polymers
Harmful effects in the production, processing and application of polymers:
 light
 oxygen
Lead to photo-, thermal- and oxidative degradation of the products
 heat
Mechanism of thermal oxidation
of polymers
1) Chain initiation
3) Chain branching
2) Chain propagation
4) Chain termination
Antioxidants:
primary (chain-breaking)  interfere with the chain propagation step
secondary (preventive)  destroy hydroperoxide groups
Stabilization by chain-breaking
antioxidants
Two reaction mechanisms by which antioxidants interfere
with chain-carrying radicals:
 Chain-breaking donor mechanism (CB-D)
phenols, disubstituated phenols,
secondary aromatic amines
 Chain-breaking acceptor mechanism (CB-A)
quinone-type compounds
stable free radicals: e.g. piperidinoxyl-compounds
Chain-breaking donor mechanism (CB-D)
The radical abstract a H atom from the inhibitor (AH) which is
transformed into the radical (A) which can interact with another peroxy
radical:
The most widely used antioxidants
are ortho-disubstituated phenols.
Stabilization by secondary
(preventive) antioxidants (1)
Decompose hydroperoxides without intermediate formation
of free radicals.
 Phospites
reduce hydroperoxides to the corresponding alcohol
and are transformed into phospates:
can react with peroxy and alkoxy radicals:
Stabilization by secondary
(preventive) antioxidants (2)
Decompose hydroperoxides without intermediate formation
of free radicals.
 Organic sulfides
transform two molecules of hydroperoxide into
alcohols:
Examples of industrial antioxidants
OH
O
OH
O
NH
HO
O
C18H37
CH3
O
2
NH
NH
NH
NH
CH3
H3C
C18H37
S
C18H37
S
S
OH
HO
P
O
O
H N
O
O
(CH2)8
O
N H
C9H19
3
Commercially used antioxidant
®IRGANOX 1010 (Ciba)
Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)
PHENOLIC PRIMARY ANTIOXIDANT FOR PROCESSING AND LONGTERM STABILIZATION of organic substrates such as plastics, synthetic
fibers, elastomers, adhesives, waxes, oils and fats.
Synthesis:
HO
OH
OH
OH
+
TRANSESTERIFICATION
Base cat., 
HO
O
O
Effect of catalyst residues and metal
contamination
In the presence of catalytic amounts of certain metal ions, hydroperoxides
decompose already at room temperature by a redox mechanism into
radical products:
The most active catalysts: derivatives of those metals, which are easily
oxidized or reduced by one-electron transfer and have different oxidation
state: Fe, Co, Mn, Cu, V, Ti (transition metals)
Much more efficient inhibition is achieved by using metal deactivators,
together with the antioxidants.
Effect of light stabilizers
Small amounts of impurities or chromophores can be sufficient to induce
photooxidative degradation even in the absence of appreciable UV absorption
by a polymer.
Types light stabilizers
UV absorbers: absorption of harmful UV radiation and it dissipation that
does not form heat. Need a thickness.
Dihydroxybenzophenones and hydroxyphenyl benzotriazoles
Quenchers of excited states: light stabilizers able to take over energy
absorbed by the chromophores present in the plastic and to dispose of it
efficiently to prevent degradation.
Hydroperoxide decomposers: metal complexes of sulfur-containig
compounds such as dialkyldithiocarbamates, dialkyldithiophosphates and
thiobisphenolates.
Effect of light stabilizer (2)
 Free radical scavengers:
analogous to that used in prevention of thermal degradation
some phenolic antioxidant can improve the light stability
Mechanism:
Hindered Amine
Light Stabilizers (HALS)
 Free radical scavenger
 Does not absorb any light above 250 nm
 Mechanism („Denisov cycle”):
 under photooxidative conditions sterically hindered amines are
converted to the corresponding nitroxyl radicals
 nitroxyl radicals combines with alkyl radicals and hydroxylamine ethers
form in this reaction
 peroxy radicals can react very quickly with hydroxylamine ethers and
regenerate nitroxy radicals
FUNDAMENTAL PROBLEM
The small-molecule additives (e. g. antioxidants,
UV-stabilizers, plasticizers etc.) can easily migrate to
the surface, thus increase their solubility and
contaminate the environment.
FOIL
SOLUTION: MACROMOLECULAR STABILIZERS
Macromolecular stabilizers
 Advantages
– Very low migration to the surface
– Long and predictable effect
– Properties (eg. solubility, miscibility) can be easily
modified by molecular weight and functionality
 Disadvantages
– Further functionalization reactions, which are
often expensive and time consuming
Macromolecular additives in the
market
Hydrolysis stabilizer
(Rachig GmbH)
Light stabilizer
(Ciba)
Resistant to acidic
environments (Chemtura)
UV and long term heat
stabilizer (Vanderbilt)
THANK YOU FOR YOUR
ATTENTION!
Thermal, antioxidative and photochemical
stabilization of polymers:
low molecular weight versus
macromolecular stabilizers
György Kasza
Department of Polymer Chemistry, Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences, Hungarian Academy of Sciences
DEGRADATION OF POLYMERS
POLYFRIEND Educational Course
September 5, 2013
Bratislava, Slovakia