Stabilization of polymers

Transcript 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 2) Chain propagation 3) Chain branching 4) Chain termination

Antioxidants:

primary (chain-breaking)  secondary (preventive)  interfere with the chain propagation step 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 OH O O NH H O CH 3 O O C 18 H 37 2 NH NH NH NH C 18 H 37 S S C 18 H 37 H 3 C S OH H O CH 3 H N O O O (CH 2 ) 8 O N H P O C 9 H 19 3

Commercially used antioxidant

®IRGANOX 1010 (Ciba) Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate) PHENOLIC

PRIMARY ANTIOXIDANT

FOR PROCESSING AND LONG TERM STABILIZATION of organic substrates such as

plastics, synthetic fibers, elastomers, adhesives, waxes, oils and fats.

OH Synthesis: H O OH OH

+ TRANSESTERIFICATION Base cat., 

H O 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) Resistant to acidic environments (Chemtura) Light stabilizer (Ciba) UV and long term heat stabilizer (Vanderbilt)

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