Stabilizer for polymers

Stabilizers for polymers are used directly or by combinations to prevent the various effects such as oxidation, chain scission and uncontrolled recombinations and cross-linking reactions that are caused by photo-oxidation of polymers. Polymers are considered to get weathered due to the direct or indirect impact of heat and ultraviolet light. The effectiveness of the stabilizers against weathering depends on solubility, ability to stabilize in different polymer matrix, the distribution in matrix, evaporation loss during processing and use. The effect on the viscosity is also an important concern for processing.

Heat stabilizers are mainly used for construction products made of polyvinyl chloride, for instance window profiles, pipes and cable ducts. Light stabilizers, for instance HALS, are especially needed for polypropylene and polyethylene. The environmental impact of stabilizers for polymers can be problematic because of heavy metal content. In Europe lead stabilizers are increasingly replaced by other types, for example calcium-zinc stabilizers.^[1]

Antioxidants

Antioxidants are used to terminate the oxidation reactions taking place due to different weathering conditions and reduce the degradation of organic materials. For example, synthetic polymers react with atmospheric oxygen.^[2] Organic materials undergo auto-oxidizations due to free radical chain reaction. Oxidatively sensitive substrates will react with atmospheric oxygen directly and produce free radicals. Free radicals are of different forms, consider organic material RH. This material reacts with oxygen to give free radicals such as R•, RO•, ROO•, HO•[1]. These free radicals further react with atmospheric oxygen to produce more and more free radicals. For example, R• + O2 → ROO• ROO• + RH → ROOH + R• [1] This can be terminated by using antioxidants. Then this reaction comes to, 2R• → R—R ROO• + R• → ROOR 2ROO• → Non-radical products[1] Weathering of polymers is caused by absorption of UV lights, which results in, radical initiated auto-oxidation. This produces cleavage of hydro peroxides and carbonyl compounds. This is because of the weak bond in hydro peroxides which is the main source for the free radicals to initiate from. Homolytic decomposition of hydro peroxide increases the rate of free radicals production[1]. Therefore it is important factor in determining oxidative stability. The conversion of peroxy and alkyl radicals to non-radical species terminates the chain reaction, thereby decreasing the kinetic chain length.

Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with organic substrate (RH) for peroxy radicals, thereby terminating the chain reaction and stabilizing the further oxidation reactions[1].

At K17, ROO• + AH -> ROOH + A• At K6, ROO• + RH -> ROOH + R• [1]

Here K17 is larger than K6, therefore AH can be at low concentrations. At low concentrations AH are more effective because the usual concentration in saturated plastic polymer range from 0.01 to 0.05% based on the weight of the polymer.

Benzofuranone is another most effective antioxidant, which terminates the chain reaction by donating weakly bonded benzylic hydrogen atom and gets reduced to a stable benzofuranyl (lactone)[1].

Antioxidants inhibit the formation of the free radicals thereby enhancing the stability of polymers against light and heat.

Hindered amine light stabilizers

The ability of hindered amine light stabilizers (HALS or HAS) to scavenge radicals which are produced by weathering, may be explained by the formation of nitroxyl radicals through a process known as the Denisov Cycle. The nitroxyl radical (R-O•) combines with free radicals in polymers:

R-O• + R'• → R-O-R'

Although they are traditionally considered as light stabilizers, they can also stabilize thermal degradation.

Even though HALS are extremely effective in polyolefins, polyethylene and polyurethane, they are ineffective in polyvinyl chloride (PVC). It is thought that their ability to form nitroxyl radicals is disrupted. HALS act as a base and become neutralized by hydrochloric acid (HCl) that is released by photooxidation of PVC. The exception is the recently developed NOR HALS which is not a strong base and is not deactivated by HCl.^[3]

UV absorber

The UV absorbers dissipate the absorbed light energy from UV rays as heat by reversible intramolecular proton transfer. This reduces the absorption of UV rays by the polymer matrix and hence reduces the rate of weathering. Typical UV-absorbers are oxanilides for polyamides, benzophenones for PVC, benzotriazoles and hydroxyphenyltriazines for polycarbonate.

Strongly light-absorbing PPS is difficult to stabilize. Even antioxidants fail in this polymer since the polymer is electron-rich and behaves as antioxidant. The acids or bases in the PPS matrix can disrupt the performance of the conventional UV absorbers such as HPBT. PTHPBT which is modification of HPBT are shown to be effective even in these conditions.^[4]

Antiozonant

Antiozonants prevent or slow down the degradation of material caused by ozone gas in the air (ozone cracking).

Organosulfur compounds

Organosulfur compounds are efficient hydroperoxide decomposers, which thermally stabilize the polymers. Sulfuric acids are produced as the product of decomposition, which catalyse further hydroperoxide decomposition.^[5]

See also

• Chemically Assisted Degradation of Polymers
• Environmental stress cracking
• Indication of weather degradation
• Photo-oxidation process
• Thermal degradation of polymers
• Weather testing of polymers

References

External links

• European Stabiliser Producers Association
• Milestab - Hindered Amine Light Stabilizers
• MPI - UV Absorbers