Stabilizers for polymers
Encyclopedia
Stabilizers for polymers are used directly or by combinations to prevent the various effects such as oxidation, chain scission and uncontrolled recombinations and cross-link
ing reactions that are caused by photo-oxidation of polymers
. Polymer
s 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.
s 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
. 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 using the 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 amine
s, 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.
Benzofuranones 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.
light stabilisers (HALS) 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 polyolefin
s, 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 .
s, benzophenone
s 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 .
gas in the air (ozone cracking
).
are efficient hydroperoxide decomposers, which thermally stabilize the polymers. Sulfuric acids are produced as the product of decomposition, which catalyse further hydroperoxide decomposition .
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
ing reactions that are caused by photo-oxidation of polymers
Photo-oxidation of polymers
Photo-oxidation is the degradation of a polymer surface in the presence of oxygen or ozone. The effect is facilitated by radiant energy such as UV or artificial light. This process is the most significant factor in weathering of polymers. Photo-oxidation is a chemical change that reduces the...
. Polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s are considered to get weathered
Weather testing of polymers
Weather testing of polymers is the controlled polymer degradation and polymer coating degradation under lab or natural conditions.Just like erosion of rocks, natural phenomena can cause degradation in polymer systems. The elements of most concern to polymers are Ultraviolet radiation, moisture and...
due to the direct or indirect impact of heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
and ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light. The effectiveness of the stabilizers
Stabilizer (chemistry)
In chemistry a stabilizer is a chemical which tends to inhibit the reaction between two or more other chemicals. It can be thought of as the antonym to a catalyst...
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
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
is also an important concern for processing.
Heat stabilizers are mainly used for construction products made of polyvinyl chloride
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...
, for instance window profiles, pipes and cable ducts. Light stabilizers, for instance HALS, are especially needed for polypropylene
Polypropylene
Polypropylene , also known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes...
and polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
. 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.
Antioxidants
AntioxidantAntioxidant
An antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
s 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
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
. 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 using the 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
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-donating antioxidants (AH), such as hindered phenols
Phenols
In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of a hydroxyl group bonded directly to an aromatic hydrocarbon group...
and secondary aromatic amine
Aromatic amine
An aromatic amine is an amine with an aromatic substituent - that is -NH2, -NH- or nitrogen group attached to an aromatic hydrocarbon, whose structure usually contains one or more benzene rings. Aniline is the simplest example....
s, 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.
Benzofuranones 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
Lactone
In chemistry, a lactone is a cyclic ester which can be seen as the condensation product of an alcohol group -OH and a carboxylic acid group -COOH in the same molecule...
)[1].
Antioxidants inhibit the formation of the free radicals thereby enhancing the stability of polymers against light and heat.
HALS( or HAS)
The ability of hindered amineHindered amine
Hindered amines are chemical compounds containing an amine functional group surrounded by a crowded steric environment. They have uses such as gas scrubbing, as stabilizers against light-induced polymer degradation, and as reagents for organic synthesis....
light stabilisers (HALS) to scavenge radicals which are produced by weathering, may be explained by the formation of nitroxyl
Nitroxyl
Nitroxyl is the chemical compound HNO. It is well known in the gas phase . In aqueous solution it acts as an acid with the conjugate base NO−, . NO− is the reduced form of nitric oxide and is isoelectronic with dioxygen...
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 polyolefin
Polyolefin
A polyolefin is a polymer produced from a simple olefin as a monomer. For example, polyethylene is the polyolefin produced by polymerizing the olefin ethylene. An equivalent term is polyalkene; this is a more modern term, although polyolefin is still used in the petrochemical industry...
s, polyethylene
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
and polyurethane
Polyurethane
A polyurethane is any polymer composed of a chain of organic units joined by carbamate links. Polyurethane polymers are formed through step-growth polymerization, by reacting a monomer with another monomer in the presence of a catalyst.Polyurethanes are...
, they are ineffective in polyvinyl chloride
Polyvinyl chloride
Polyvinyl chloride, commonly abbreviated PVC, is a thermoplastic polymer. It is a vinyl polymer constructed of repeating vinyl groups having one hydrogen replaced by chloride. Polyvinyl chloride is the third most widely produced plastic, after polyethylene and polypropylene. PVC is widely used in...
(PVC). It is thought that their ability to form nitroxyl radicals is disrupted. HALS act as a base and become neutralized by hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric 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 .
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 polyamidePolyamide
A polyamide is a polymer containing monomers of amides joined by peptide bonds. They can occur both naturally and artificially, examples being proteins, such as wool and silk, and can be made artificially through step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,...
s, benzophenone
Benzophenone
Benzophenone is the organic compound with the formula 2CO, generally abbreviated Ph2CO. Benzophenone is a widely used building block in organic chemistry, being the parent diarylketone.-Uses:...
s for PVC
PVC
Polyvinyl chloride is a plastic.PVC may also refer to:*Param Vir Chakra, India's highest military honor*Peripheral venous catheter, a small, flexible tube placed into a peripheral vein in order to administer medication or fluids...
, benzotriazoles and hydroxyphenyltriazines for polycarbonate
Polycarbonate
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...
.
Strongly light-absorbing PPS
Poly(p-phenylene sulfide)
Polyphenylene sulfide is an organic polymer consisting of aromatic rings linked with sulfides. Synthetic fiber and textiles derived from this polymer are known to resist chemical and thermal attack. PPS is used to make filter fabric for coal boilers, papermaking felts, electrical insulation,...
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 .
Antiozonant
Antiozonants prevent or slow down the degradation of material caused by ozoneOzone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
gas in the air (ozone cracking
Ozone cracking
Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking...
).
Organosulfur compounds
Organosulfur compoundsOrganosulfur compounds
Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odours, but many of the sweetest compounds known are organosulfur derivatives. Nature abounds with organosulfur compounds—sulfur is essential for life. Two of the 20 common amino acids are...
are efficient hydroperoxide decomposers, which thermally stabilize the polymers. Sulfuric acids are produced as the product of decomposition, which catalyse further hydroperoxide decomposition .
See also
- Chemically Assisted Degradation of PolymersChemically Assisted Degradation of PolymersChemically assisted degradation of polymers is a type of polymer degradation that involves a change of the polymer properties due to a chemical reaction with the polymer’s surroundings...
- Environmental stress crackingEnvironmental stress crackingEnvironmental Stress Cracking is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. Environmental stress cracking may account for around 15-30% of all plastic component failures in service.ESC and polymer resistance to ESC have been studied...
- Indication of weather degradation
- Photo-oxidation process
- Thermal degradation of polymersThermal degradation of polymersThermal degradation of polymers is molecular deterioration as a result of overheating. At high temperatures the components of the long chain backbone of the polymer can begin to separate and react with one another to change the properties of the polymer...
- Weather testing of polymersWeather testing of polymersWeather testing of polymers is the controlled polymer degradation and polymer coating degradation under lab or natural conditions.Just like erosion of rocks, natural phenomena can cause degradation in polymer systems. The elements of most concern to polymers are Ultraviolet radiation, moisture and...